If by that you mean can he sign everything as though he’d installed it, no, it would be illegal, because he didn’t.
If you HAVE the situation of a boiler you need to be made “legitimate” then a Registered engineer technically would have to remove and reinstall it, and flush clean and refill the system with inhibitor. Then he could say he’d installed it, notify it to GSR for Building Control, and fill out the Guarantee forms.
Also be aware that if you don’t have all the paperwork in place, you can expect to find that manufacturers’ warranties, and service contracts such as British Gas 3 Star, are void.
There is more here, issued by Corgi to registered engineers.
(You need to download the file (3 pages), preview doesn’t work.)
The engineer is responsible for the fixing on the wall, so technically it would heve to be removed. If he decides he doesn’t need to physically remove it, and could check the flue etc in situ, and can be satisfied that the system has been cleaned and chemically treated appropriately the amount of work is much less. You may well find an engineer prepared to do this, but as it’s the “expensive” part of the job, it wouldn’t be on normal hourly rates.
ChrisR