The move from Pipex Homecall to TalkTalk if you do nothing cold go wrong, as could a move to Sky, which do gamble on?
TalkTalk in terms of speed will pretty much be the same as the Pipex, and the move might actually be very smooth, depends on how old the pipex hardware you are on is at the exchange.
Both services will be full LLU, i.e. rental and BB via the Talktalk or Sky hardware, so a toss up.