Don't Welsh-speaking drivers ever drive into England?
Of course they do and I'm sure they realise that Welsh does not have official recognition in England (and Welsh speakers do not form a cohesive, contiguous community in England). I think this argument is flawed - do Italians expect to see Italian-language signs when they drive in France, for example?
What about the Urdu and Hindi speakers in the UK who probably outnumber Welsh and Gaelic speakers? Could we find some room on signs for them too?
Urdu and Hindi are not endangered languages and don't need to be promoted in the same way. There could also be an argument made that they are not indigenous to the UK, so shouldn't expect the same level of backing (I'm not saying that I agree with this - just presenting a possible viewpoin).
If you spend any time in the London borough of Tower Hamlets, you'll see plenty of bilingual street signs (English/Bengali) and passing through Southall station near Wembley the other day I noticed that the station sign was written in English and Punjabi. It's possible - I don't know, as I've never been to either - that there is some bilingual signage (English and Urdu or Gujarati, for example) in places such as Bradford and Leicester.
I'm all for cultivating minority languages in regions where they are spoken, however, I'm not fully convinced that road signs are an appropriate medium.
Road signs are just one element in the promotion of minority languages - not the be-all and end-all.