You are not committed to going ahead at this price or any price. Nothing is sealed yet.
You could wait for the valuation....but I suspect you have decided you over-offered anyway and will still feel like this regardless of valuation. The thing is, you don’t absolutely love this house and now, at this point don’t feel you want to pay what you offered.
You need to decide if you actually want to move to this house first of all. Is it the house you want or do you need to just pull out because it’s not.
And if it is the house you want, what are you prepared to pay? Will you pay what the valuer puts on it or is there a chance you aren’t even willing to pay that or is there a possibility of you meeting mid way between a lower valuation and what you offered. This is what you need to decide.
It does sound like the survey and the valuation will be useful to you and help clarify your thinking. Until you have those, you perhaps don’t know what you’ll be prepared to pay and won’t have any basis to go to the vendor for further negotiation.....so wait for that info.
Once you’ve got that info, make a decision about a range within which you will pay. Go to the vendor and make the lower offer explaining your reasoning. And then see what happens.....they might turn you down out of hand and the sale falls through, or they might negotiate and push you to your new upper limit or they might just say yes.
I don’t think there’s a problem in renegotiating in light if valuation and survery. The market is changing fast and vendors might not like it but they need to expect it. To be honest, buyers are foolish to ignore something like a lower valuation or falling prices or problems in a survey. People shouldn’t just be looking for a lower price for no reason after agreeing a price, but that offer is very provisional and on the basis of valuation and survey....subject to contract. There is no moral obligation to complete at a price you offered which turns out to be wrong.
So, to start renegotiating now without more info might be premature and fruitless. Wait for full info and then consider again and renegotiate if it seems right then. It’s a business transaction and you do need to look after your own interests and nothing has been committed to u TIL exchange. This period is for gathering further info and prices often adjust to reflect it. Those who get furious about this happening don’t appreciate the workings of a market and are too personally involved and lacking objectivity, in a way which isn’t helpful.