I'm from Cardiff, It's a beautiful city and admittedly it does have a few big tech companies to work for. There is a decent polish community there too.

However, Cardiff is not a big big city, so it doesn't have hundreds of tech companies, and you might hit a pay ceiling faster in Cardiff than you would in London, however, the cost of living is also lower in Cardiff than London... Swings and roundabouts. Bristol is an hour or so's commute though with more tech firms.