Has anyone played as the UK? I've owned this game for six years now and still can't bring myself to play them. Managing all that empire on day one just seems like too much of a chore.
One thing that does help are the oodles of puppet states which help in India that help you keep the region under control. Though, particularly, it depends on the route you wish to go. The hardest thing is balancing your SoI and knowing who you really want in it, and who you're willing to give up. It's really that kinda deal, honestly, as well as how much you want to keep up with and how much you want to handle.
Let's look at the Americas:
The easy route is to not take anymore territories in North America so you have no conflicts with the US - while you can lose Canada in a war, you don't want the US in a war with them launching war objectives to take Canadian states. Oh, sure, taking Panama is a good idea, along with maintaining your territory there, but that's not so terribly difficult. You don't really need anymore - taking one of the Guyanas isn't really necessary. Sphereing a large portion of
South America isn't a bad idea, especially Colombia/Venezuela/Brazil, as you'll have a land border with all three due to Guyana and the Canal. That is easy enough to maintain - just be careful of butting heads with the US and you'll be fine. Basically, here, this is easy mode - don't stick your neck out
too much, and take what is easy, and don't provoke the US and you won't need to put much in the way of the forces here.
The hard route in the Americas is to try and break the USA. Ally with Mexico before they are invaded and lose the west and use your superior resources to eliminate their armies early on. Take a few of their major cores, and constantly invade afterwards to take colonial states on the cheap. Make sure to occupy the country - also, only take states in the North, naturally. You can take New England or force its release as an independent nation (or conquer and release as a puppet). Make sure that, when the CSA is released, that you invade as soon as possible to support them in their war. That way, you reduce the US to a minimal power - eventually forcing to secondary if you're successful, and you're able to sphere a stronger Mexico and the CSA as well as the remainder of the Americas.
Mind, keeping the US down will be a bit of an investment - you could be exercising this later on. It's fairly easy to use that time to expand elsewhere.
Asia is always a tempting choice. Johore isn't a bad idea, as it secures your possessions in Malaya and allows you to position just one army there. It might be a good target at the start of the game. Locking up more of East Asia isn't a bad idea either - Brunei, Bali Lombok, and Indochina also give you more and more states for you to take advantage of. It's, of course, an option to start taking provinces from China as well. If you so desire, it isn't impossible at
all to take all of China, Indochina, and India. That is, if you want to game your war declarations to get the minimal amount of infamy for taking colonial land. Save and reload is your friend.
At the least, do secure Malaya, Burma and other neighboring possessions so your territory is completely contiguous. If you want to recover the various princely states, never take the Empress of India so that the Doctrine of Lapse event keeps triggering. The more you lose of the princely states, the slightly stronger your interior states and, more importantly, the fewer nations you have to worry about managing the SoI for.
Another option is to start planning to lock up as much of Africa as possible. In general wars against the various European states that you're liable to wander into, start nabbing colonial provinces to reduce their colonial range - it'll make it easier for you to secure the continent and to prevent the Europeans from getting into the initial colonization rush. Take the native states on the exterior first before France poaches Algeria, etc. Either support Egypt and keep it strong or conquer it as well. Continue to build ports and make sure to build as many ironclads as possible in order to support a colonial expansion in Africa the moment you can. Take Sokoto as well, as the US seems to be fond of claiming it in their SOI by my experience.
Eventually, you should get an event to conquer the Boer states (I say should; I never got it, and it is random). So that should be a focus as well - those territories are a good location to become states late game. If you're patient and have the time, you can always justify a casus beli to conquer them, and just reload until you can take for minimal infamy. It'll take a long time, however, but it is possible. Other tha
Basically, for Britain, you need to decide who you are ok with growing and expanding, and who you are going to make a point in neutering from the start. They are the nation that is least dependent upon good events or things just falling in place - you can plow through your plan. Say you want to keep Greece in your SOI - you're gonna need to fight off Italy it and never take any Greek cores, such as Cyprus or Crete. Realize that, without a land border with most of Europe, you're gonna be having trouble maintaining SoI against a committed continental foe. Spain (Gibraltar) and Portugal (only borders Spain) are good continental spherelings, mind - unless you decide you want their colonies instead. It's very hard to maintain Belgium, for example, and the Netherlands. So, unless you want to get tied up in Europe...
Granted, you could always try and chomp off a bit in land in Europe as well and go hog wild as well - but remember, your economic strength will be overseas, and you have to do enough to preserve that.
As for the question of releasing Canada/Australia/New Zealand/etc, that's up to you as the player. Mind, if you're efficient with saving/reloading, you might not need it - even then, you should only need it if you're taking a lot of land. All of those regions easily become states, which does help your industry significantly. On the flip side, the regions would likely be stronger if you let them go (more immigration, if I have my mechanics correct), but then you have the issue of other nations taking them from your SOI (principally the US) so you have to make that decision. Still, it isn't really necessary.