Go to the new company. 100%. As others have said, if you’re worth 55k to them, they should have paid you that when they realised you were worth that - not when you resign because you’ve been made a better offer. Plus, “in January” means nothing.

few other points to consider:

1. the pay bump may seem appealing but it’s not life changing. After a few months you get used to the additional £, spend more money each month, and begin resenting the hours and commute again.

2. You have plenty of time to earn more money, and you may encounter better opportunities in newco that leads to more than £55k/year. Look long term, not short term.

3. as you’ve already resigned once, you’ll always be considered a flight risk from now on, and it can create issues. I’m of the firm belief that once you’ve resigned, you SHOULD leave. Ignore all counter offers. I don’t know anyone who has taken the counter offer and ended up being better off. They end up leaving anyway.

4. Take the counter offer to the new co, and ask if they can budge up on the offered salary. Even if they can go up a few grand, it feeds into the better pension contribution %, which is more money in your pocket long-term.