http://www.ipo.gov.uk/types/tm/t-os/t-find/t-find-prop.htm
If they hold the mark, I would re-brand. If they do not, I would try to launch as soon as possible with an MVP.
However, just bare in mind, even if you are 'right' and you are not in violation of their trademark, if they have the money there is nothing stopping them from taking you to court after they have tried amicable means to solve the dispute to no avail.
Just to give you an example, I learnt the hard way that even when you are right, it is often better to settle than spend thousands trying to fight - especially if the other party has deep pockets.
I had a legal dispute before where I was legally in the right - 99% chance of success in court, but we decided to settle as going to court usually means everyone loses but the lawyers. It's also pretty stressful. Pick your battles.
If you decide you'd like to keep the name, then don't worry. These guys will have to get serious and actually follow legal protocol before even filing for any case. Such a case would be held in the high court, and this will cost them tens of thousands.
What they have sent you does not appear to be a protocol letter and if they do send you one, they must explore resolving the dispute amicably before they can file for a case. You will get at least 30 days after receiving a protocol letter to respond.
I'd check out what exact trademarks they hold and research a bit into case law surrounding this. Find out if they have a strong case, if they don't, I highly doubt they will spend tens of thousands on taking this to the high court.
Just my 2 cents, i'm not a lawyer.