Defer entry

I have been offered a medical defer entry by uni x and i accept it . But i am interested in uni y so can i put my defer entry on hold of x and apply for uni y the following year? Is this possible.

If you apply next year your place at X automatically becomes void.

Not quite, they wouldn't be automatically released from their current offer, they'd have to request it from the Uni involved before they could be considered elsewhere.

But yes, the short answer to the OP is: No, you can't.

AFAIK if you make a new application your old one is no longer valid.

It stays valid until you're released by the offering Uni. We have to do lots of releasing at the start of each admissions cycle when people realise they can't apply elsewhere whilst holding a deferred offer. The effect is the same though, they can't hold the deferred place as a banker, they have to apply again.