I'm so sorry about your rabbits. For any future rabbits you may get, there is a vaccine for Myxomatosis available in the UK.
I made my rabbits 'juice' before with fresh fruit in a juicer, and gave it to them watered down as a treat. I think if you gave them extract from rabbit safe fruits it would probably be okay, but keep in mind that fruit is really more of a treat for rabbits. A lot of them are high in sugar and should be fed sparingly.
Why not just give them small pieces of fresh fruit, fresh greens, stuff like that? I'm not even sure the extra vitamins would make any difference, and fruit juice has added sugar and such that could possibly mess with their GI tracts. You could always call your vet and ask them their opinions. If they are already treating your rabbits I'm sure they wouldn't mind answering your questions.
So sorry again that your rabbits are sick! I hope they can pull through and keep us updated!
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