I've just recalled another UK Invaded novel -
Rule Britannia by
Daphne du Maurier. Not the greatest in the genre but of some interest for the different plot. Very much a What-If idea novel.
Following the collapse of major economies in Europe the UK is struggling with 1970's hyper inflation, union strikes. A deal is done to join the US as 4 new states (I think) and the US send in the troops to consolidate the new union. Aircraft carriers and Assault ships landing US Marines etc.
They are more of invaders and occupiers than the friendly GI invasion of WW2. The locals don't like all the changes and hit back in their own way. One of the US ships explodes and sinks out at sea and it is hinted that the Royal Navy are fighting back, Princes Charles and Andrew have landed to lead the rebellion....
There is another book whose name I have just remembered -
Division by Graham Ison
It's a late 1990's political novel. The premise is that Britain joins the new European currency - ECU. There are major protests against this (think Brexit!) and the situation is made worse by prices appearing to rise higher than they should. Riots across the UK, Govt dithering, assassination attempts, collapse of law and order. The EEC/EU decide to act and send in the troops across the Channel to restore order under the terms of a new treaty.
Bizarrely the British Army not only let them in but they also let the German Panzer Division stand guard at Buckingham Palace...
It ends with a new UK Govt and the utter disbandment of the Irish Guards because one of its men tries to shoot the Queen
Again, the invaders are invited in. This seems to be the theme for any takeover of Great Britain.