Loads of experience of TUPE, I was a union rep in local government at a time when they were outsourcing half the council. Only had it done to me in the private sector though...
All TUPE means is, your conditions at the time of transfer are protected. Doesn't directly protect pensions, they just have to be 'broadly similar'. Any changes they make subsequently that are 'connected to the transfer' aren't allowed. Lawyers go skiing in Klosters on the money they make out of 'connected to the transfer', as the wording is that woolly. And if you sign a 'new and improved contract', all bets are off.
I did come across people that did sign a new contract and for them the sky was the limit, but they would have done well wherever they were.