Luca, I wouldn’t let the seller’s preferred speed turn uncertainty into an inflated offer. Comparable sales can inform the price, but differences in condition, tenure and maintenance obligations may limit the comparison. Ask the lender and conveyancer what can realistically be completed within...
Before applying any percentage adjustment, I would confirm whether 430 sq ft is genuinely the total internal area and how five beds are configured within it. That combination is unusual enough that a simple price-per-square-foot comparison could mislead.
After that, lease length and service...
What loan amount sits behind the £651,300 purchase price? Without the deposit and resulting LTV, nobody can tell whether the quote is genuinely poor or simply priced for that tier. Also, are you paying the fee upfront or adding it to the mortgage?