The thing about the perception of accents is that it is extremely opinion based. Some people love French accents, some people hate them, much the same way as some dialects are loved by some and hated by others. It is safe to say that everybody will have a personal opinion on how British (or American or Australian/others, although these last ones are decidedly rarer) accents of German sound and that you will receive the entire spectrum from love to hatred when asking enough people.
That said, none of the accents really inhibit intelligibility. Most sounds of the English language are fairly close to their German counterparts so the phoneme mapping is rather good and not much gap-filling remains. Yes, /x/, /ç/ and /r/ have no direct English equivalent, but usually the sounds around them provide enough clues as to which word is meant. There are languages out there that are much more removed and whose speakers’ accents are generally much harder to understand. (But also note that it is possible to learn how to speak accent-free German no matter what your mother tongue is.)
What might inhibit understanding is bad choice of word order (depending on how far away a sentence is from what a German expects) and funny emphasis on certain words — and occasionally mispronounced foreign words when pronouncing them in a more German fashion. (/baige/, anyone?) All things considered, it is usually nothing that can’t be resolved after a single ‘excuse me?’
