I see no reason why you couldn't, your UKCAT score and details will be stored by Pearson Vue until universities retrieve it after the UCAS deadline. If you decide to take your offer it'll just be a case of no universities retrieving your score, but no universities you've applied to this year will know, and even if they did they couldn't retract an offer.
But seriously, i can't for the life of me think why you're even considering this option. Applying for medicine this year must have been one of the most, if not the most, stressful experience I have ever been through. It's so drawn out and you're constantly left hanging, knowing that at any point you could be rejected by the smallest of margins. You say you have a strong application apart from your UKCAT this year, even with a higher UKCAT next year you may come out with 4 rejections. Surely you must have realised the extent to which applying for medicine is like a lucky dip? I have a friend that has better grades, more work experience, a similar UKCAT and an impressive personal statement. She was rejected by all 4 pre-interview while I have 3 offers; if we applied next year who's to say that these roles wouldn't be reversed?
So yes you can, but you'd be taking an incredible risk and wasting a huge opportunity.