The deposit is mainly for the benefit of the seller; in case you default halfway through and he's run up costs. He would keep the deposit. It's not uncommon to "exchange contracts", and "complete" simultaneously. No deposit is needed as the whole price is paid in one go. Lawyers don't usually like doing it this way though.
10% is a convention. It's always negotiable. Occasionally, a reduced deposit is agreed where buyers may be struggling to find 10%.
Although claiming via NHBC is often a huge hassle, it's still an insurance policy against defects. The only survey you might need is a simple valuation; to make sure you're not paying too much.



