Broadly agree with what the others above are saying - but let me explain the technical reasons why.
In doing such a loft conversion project, there are 4 (main) parts you have to satisfy for Building Regulations:
The structural strength
The insulation performance
The exit routes in case of fire
Ventilation
You seller has probably done the first one OK. However to demonstrate that he has done it, he would have had to submit the calculations to Building Control and he hasn't done that. So, no way, Jose.
The second one you may be able to inspect. To satisfy BRs there are minimum standards of insulation set for good reasons - the Government wants UK to use less energy in our homes, and - it will cost you less to heat to a reasonable comfort temperature. The way this is done is to install insulations between the rafters - however the amount of insualtion required is considerable. Until about 4 years ago a product called Actis Tri-ISO was in use. It is a multi-foil product and very thin - which made it great for this application where thickness was a real issue. Then there was doubt under tests that it would work the way intended so some local authorities BC banned its use. I am not up to speed on the current situation. The alternative is that at least 200mm of Celotex insulation was installed. If you don't have one of these two fitted the room will be expensive to heat - and does not meet BR.