It's hard to know what's wrong with your burned and shrivelled one: it could be drought, it could have drowned, could be other things. The one that is too big for its pot, better pot it on to a bigger pot then.

Ugni molinae (Chilean guava) (flambeau is a variegated form) doesn't do well out of doors in SE England. I've had one in the ground for several years and it barely flowered, fruit never ripened, it suffered in dry weather, and got frozen back each winter, in general got smaller and smaller each year, until a brief spell at -10 killed it completely. It grows naturally in the very wet climate, with mild winters, of southern Chile, more like NW Spain in its climate than UK. To grow it in a pot you need ericaceous compost and keep it moist (without drowning it) using collected rain water (or tap water if you are in a soft-water area, unlikely in Berkshire). I believe it can be grown well in south Devon, and was cultivated there in the 19th century: Queen Victoria considered it her favourite fruit. I'm surprised they haven't caught on given the present fashion for eating berries - they are like a very fragrant blueberry.