On the Welsh thing - I'm English and have had a couple of experiences of people being a bit funny. I lived in Chester and went into Wales once in a while on work and one day popped into a newsagent outside a train station asking if they knew the train times, as the timetable was nowhere to be found. They were chatting in english about TV shows or whatever and instantly went into welsh and claimed to have no knowledge of the station. Weird as it was likely their chief source of income and the trains were about once an hour. Oh well.
I lived in Scotland for four years and had no trouble at all, plenty of ribbing but always good humoured. On once occasion I was threatened with violence for being English - at a funeral wake of all places but the person was extremely drunk and extremely upset and looking for a fight. I could have had on glasses he objected to or anything - it was just a reason to lash out. I did once see a student beaten half to death for admitting to being English to a gang of football hooligans. Again, if he'd said he was Scottish he would have been from the wrong town or whatever. People will hold it against you if they are after trouble but the rest of the time people live and let live. Curiously, some of the most rabid nationalists live right at the border, but then again in Carlisle, Bamburgh, etc. you can find raving English nationalists too.
Race is a thorny issue in much of Scotland though - was on a driving tour with a Nigerian friend and some thugs came over with a camera and told him they were taking their photo with him as nobody would beleive they'd met a 'real nigger'. They didn't ask, they told us it was happening. It was clear that we weren't free to decline. You don't have to get very far out of the towns for things to get pretty rustic in Scotland, but it's much like Northumberland, East Anglia, etc. in that respect. Places with that 'long way from anywhere' vibe. In the cities you could be anywhere really and nobody bats an eyelid.
Don't let it put you of going, just apply a modicum of common sense.
For the record, I've lived in 12 cities around the UK and by far the most peaceful place I've ever lived is Leicester. Gem of a place, over 25% green spaces all open to the public, two great universities, very good commuter links to everywhere, London is a shade over an hour away and very, very cheap cost of living. Cosmopolitan, all the mod cons and the bad areas are discrete and easily avoided.