You actually shouldn’t keep them charged, they need to be “storage charged”. You’d want them at around 50%, for longevity, in storage.
Having a quick look at the package they sell, you can already tell that it’s not as simple as “unplugging cells and putting new ones in”. A gifted electrician might be able to order the correct cells and replace the over-discharged ones, but as a layman, you shouldn’t try. Lithium Ion cells are no joke, a mistake can lead to spontaneous combustion (puncturing their wrap, for example).
If that option doesn’t exist for you, the only other one would be to get a new battery pack. The real issue is the price there, the fact that they don’t send to the UK can be circumvented by “parcel forwarding”, there’s webpages on the internet where you can create an account, and get introduced to people in the US who offer to forward your parcel internationally (for a fee).
Here’s a few examples.
https://www.stackry.com/en/home
I’ve used stackry for a remote controlled car in the past.