I'd do it personally, and keep a mental eye on it for a whole (a few years=) to be aware if it was more contankerous than you'd expect. My home WiFi/ADSL modem router located in a non-airconditioned dining room (never under 5C never over 30C) crashes mainly one monthly and an 8 port switch maybe every 6 months.
I'd expect it to be OK if you were sensible. After all, how hot can it get in England? :-). Seriously, if it was not insulated more than minimally I'd expect heat not to be an issue short term. Long term, electrolytic capacitors will halve in lifetime per 10 C rise in operating temperature. It may not use ecaps and you'd hope none were stressed highly - so not very hot so long life.
If it had minimal but some insulation from surroundings (say plastic case that stops convection and gives own heat a very mild barrier to cross) it should take the edge of British winters. Batteries dislike sub zero temperatures usually. Probably OK for backup batteries where power is low.
I'd not expect condensation to be much more of a problem than elsewhere. If you looked in the attic and there was a glass tumbler there - would you expect it to ever be fogged with condensation. Local conditions may make that ayes but usually I'd expect no.