Why did Virgin Media merge with NLT: Telewest?

I would suggest they just wanted a bigger market share to get closer to domination of the UK cable/telecoms market. You can do this by out pricing/performing your competitors and getting more custom (and thus more sales/profit). Or if you have an exceptionally rich man behind your company you can offer the owners of your competitors money to buy their business or "merge" with you. I don't know the details but suspect that is what happened