The main names to be aware of are
Halifax + Bank of Scotland = HBOS (also include Birmingham Midshires and 45% of Sainsbury Bank)
Royal Bank of Scotland includes Nat West, Ulster Bank and 50% of Tesco Finance
Barclays owns The Woolwich
Citigroup includes Prudential and Egg Banking
There are others, of course. Most banks have joint ventures and shareholdings with some of their "competitors" but a small stake or joint venture doesn't make them the same group.