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There already will be a penalty if the seller pulls out prior to completion. It should be the amount of your deposit, but could vary.
I think you would be very lucky to be moved in before xmas anyway. 2 months from offer accepted to completion is pretty quick in my experience. I'd say late January would probably be a fair expectation for completion. Their wanting to complete in March isn't that much of a stretch, and with a baby on the way it's completely understandable. If you really want the house I'd cut them a bit of slack, be a little bit human and go for it.
I'm sure it would be technically possible to agree the sale on a floating index (house price indices certainly exist). They definitely won't agree to it mind.
Appreciate that as a buyer you'd risk forfeiting your deposit if you pulled out after exchange - but what would the seller risk if they did the same (between exchange now and completion in March).
I can't imagine they'd lose that much, so the risk in this situation is seems very asymmetric. You should, at least, demand a premium for that (possibly in the form of rent or your own deposit from them). Again, they won't agree to this either.