Well, I'm not sure a double-gun device would work. Presumably that involves firing two components at each other, or alternatively at a target placed between them?
I would assume two components at one another. Most likely it would be dropped to a single gun when the closing speeds required are better understood - the double gun gives you twice the assembly speed for minimal extra complexity.
Also, they might have gone down the path of the plutonium gun type bomb, but that shouldn't be too much of an issue since my understanding is that a top British explosives scientist was loaned to the Manhattan Project to figure out how to make the implosion devices work.
This was uranium, not plutonium - plutonium wasn't even considered at the time the decision as made (it had only just been shown to be fissile a matter of weeks before). A Pu gun-type bomb (thin man) is unlikely to work, but since an implosion type device is much more efficient with scarce fissile material I think it's very clear that they'll look into it at some point.
Is there a British equivalent to the United States Department of Energy site? It could be useful for nuclear history research.
http://nationalarchives.gov.uk/cabinetpapers/themes/nuclear-power-programmes.htm
Have a dig through the entire national archives site - quite a lot has still not been digitised so you need to pay for it, but just about every government document not still classified is in there and hence available. Cabinet minutes and the like are in there too, so you can see who said what when the decisions were taken.
Another option would be for use as fuel by breeder reactors. Apart from MOX, there isn't a way to make a conventional reactor run on plutonium. There was an experimental reactor in the United States that ran on molten plutonium though.
No reason in theory you can't run a reactor on only Plutonium, but the engineering is going to make it rather different to a conventional Uranium one. The MSR experiment tried running on pure Plutonium for a while, apparently, and I think some of the breeder reactors are designed like that too.
Also, it's interesting that the British wanted to go from gas cooled reactors to water cooled reactors. It was somewhat the other way around in the United States. This might be because at the time gas cooled reactors were less efficient then water cooled ones because they ran on natural uranium and had to use gas to heat steam to drive steam turbines. Later on it was discovered that gas cooled reactors can actually be quite efficient, as they can drive Brayton cycle gas turbines, which are more efficient than Rankine cycle steam turbines.
It's all about the temperatures - Magnox ran at about the same temperature as PWR and CANDU. AGR got to higher temperatures and hence better efficiency, but at the cost of requiring enriched fuel. The drive was all about increasing the heat rate, i.e. the efficiency of the system, and I think that was due to the very early Magnox design (this was written while Calder Hall was being built) having to run at relatively low temperatures to keep the magnesium fuel rod material to specification. AGR changed the material, but this is what forced them to use enriched fuel.