FTB buying converted flat

hjd

My son is buying a flat in a building converted from offices.
Mortgage broker recommended Nationwide.
LTV under 50%.
Mortgage application filled in and accepted on NW system 22/04, he received text to confirm.
Text received Tues 3/5 to say valuation will be today (5/5).
Does this mean they are OK with the application or do they not do checks till after the valuation? I have found the NW flowchart but it doesn't make that very clear. Should all be OK - no loans/debts other than student loan - but he does tend to spend all his money each month! This flat came up before he had had a chance to "sanitise" his bank statements.
Vendors want exchange asap (in 28 days from receipt of contracts if possible). Completion not till September. Is this a normal schedule for exchange on a new build?