That boiler construction does not prevent its testing and use under the MELG scheme, but it would have to be tested very carefully as if a "new" boiler, and I think some club boiler-examiners might be reluctant to test it.
The bits visible in the photos that worry me are –
– the screws holding the dome-ring to the shell, if they are tapped directly into the copper, despite the solder caulking; and
– the solder's quantity and "cauliflower" appearance on the firebox stay-heads, which suggests the builder had great difficulty caulking them properly.
'
Other than that the general standard of work looks as it should be; so it might not be a write-off even if you need make a new boiler for it.
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Gauge – I concur with Dave Wootton that this is 2-1/2" , having measured a peg and the photo on the screen.