Your solicitor is doing her job cautiously & properly. If you buy without proper evidence that any extensions etc were done in accordance with building regulations and planning law, any enforcement issues that arise subsequently will be your problem, not the vendors.
If the certificates aren't forthcoming it may not be a showstopper, depending on the seriousness of the issue. I had a situation buying a house where the seller had put in a Velux which needed planning permission. Our solicitor made the purchase conditional on the vendor getting permission to retain the window at his expense. A kitchen extension is a bigger matter, and if the vendor can't satisfy your solicitor that it's kosher, you might be as well to pull out. As you say, it could be a problem if you were reselling, not to mention right now with your mortgage lender.
Don't worry about having signed the contracts. Your solicitor won't return them to the seller's solicitor unless she's satisfied that everything's in order. Until the contract is returned to the seller, it's not binding on you, even though you've signed it.