Suitable LMS parts - depending which variant one would want to do - are available from Comet, 5522 & Bill Bedford at least.
The 'easiest' way would be the Mailcoach LNER version, which is a plastic kit.
The Roxey ones, as you say, would be best for an SR based layout though. My knowledge of the SR is minimal, so I dont know whether using replacement sides and end with the Hornby SR offerings is a possible way forward? The Hornby model may not sound cheap, but if you look at the cost of full etched brass kits and the time taken to build them it may be a cost/time effective solution.
HTH in some way, shape or form.
Cheers,