You'd have to ask them if you can be released from your contract. There is an early termination fee, which you would need to have them confirm based on your individual contract.
Their contract is very vague, as I expect are all ISPs. Unless your service is hard down for an extended period, you will definitely be subject to their exit fees.
19.6 We can’t promise you a consistently fault-free Service but we will use all reasonable
efforts to reduce the effect of any interruption to it.
19.7 Sometimes we may need to interrupt your Service to maintain, repair, upgrade or
make technical changes to it. We’ll try to let you know before we do this and we’ll restore
your Service as soon as we can.
Whilst issues like this one are annoying, their uptime and performance is very good IMO. I've had maybe 10 days or less in total where there has been packet loss at peak times since I signed up in May 2022. The service has never been hard down for me, other than maintenance in the early hours of the morning.
I work from home at any/all hours managing a team that covers production support, so I do have a backup FTTC connection with TalkTalk as I can't afford to be offline at any time. It costs me £1 less than my 1000/1000 HB connection for 150/25 and always has very spiky latency (little packet loss though). I had the same issue with Plusnet in the past and suspect it would probably be the same with any other provider as it all uses the same infrastructure.
Even taking the consistent 900+/900+ bandwidth out of the equation., if I had to choose one over the other, I would definitely have the 99% stable HB connection over the spiky performance of the FTTC one.