You can pay taxes early. Go to your Self Asessment home page and choose "Make a payment" (direct link once logged in).
Then pay whatever you want to. It'll initially just end up sitting on your tax account not allocated to any particular liability. Once you submit your tax return, it'll be deducted from whatever you owe. And you can request a refund at any point, either because you do want the money back after all, or because you turn out to have overpaid after you submit.
Source: personal experience of making a small payment in September to see what would happen - it got taken off the bill when I submitted my return in December. I haven't tested the refund bit but it looks pretty clear it should be possible.