Sourdough Bread In the UK?

I do like sourdough bread but it doesn't seem readily available in the usual supermarkets here in the UK. I'd like a sliced wholewheat flour bread however the only wholemeal flour one I can found is unsliced. I have found a sliced sourdough here but it uses wheat flour (not wholemeal) and is quite expensive. Just wondering what peoples experiences of finding sourdough in the UK are?

Sliced Sourdough

Waitrose do a sliced loaf

Ocado, online, have a variety of different sourdough breads (the olive one is very nice)