Came across this thread a while ago and thought I'd sign up and add my experience from last year with seat legs that may be of help!
We've got a set of the GWR single seats used as an upcycled table set in our dining room (not the GWR 4 seater table though as figured that's a wee bit too small for several adults at dinner time). Also have a couple of the small single tables sat in storage but not yet done anything with those! One has the power functions, the other hasn't. For not much more than the cost of a half decent generic oak dining set off the high street this looks the bee's knees, seems to generate a load of interest and will probably last as long if not longer with a wee bit of care.
We contacted a local fabrication company who came out, took measurements and did a CAD design mock-up before creating a new pair of quality heavyweight steel legs for each seat finished in satin black before fitting (a brown colour match was several times the price and in any case the black suits it equally well). The legs have a bit of a ski effect in that they come out slightly from the base of the seat but do not protrude any further than the furthest point of the rear of the chair. I can get better pictures of the fitting but the attached should give a rough idea of the end product and you can probably spot that the single brown leg that came with the seat was not used so I have several spares of these. The result of this leg design is an extremely secure seat which has absolutely no chance of toppling over, doesn't wobble nor requires securing to the floor.
Unfortunately the company who fabricated them made a very minor error with the measurements as when the seat is reclined fully the top of the legs catch the bottom of the seat shell ever so slightly (which makes a loud metallic rubbing noise when you inevitably move about) and with the only solution to remake them with an adjusted angle which they didn't want to do we've ended up not paying anything for them as is. Shame as I suspect they'd have got several more custom orders from a revised template and a few recommendations! The design though works in principle for both the single seater and twin. In any case the noise issue has been remedied by some generic felt furniture pads for a mere £2 from Amazon applied to the top of the legs which aren't visible and completely removes the noise at the expense of perhaps 2 or 3mm of recline. I may get them redone one day but they're fine for now, look great and are perfectly useable as is.
The only other point to think about is if replicating the height of the chair as it would have been on the HST itself it's a couple of inches too low for either sitting comfortably at a standard desk or dining table unless you're extremely large! Not a problem of course if you're using a matching GWR table and use the height that was setup for. Happy to share the various measurements on mine though - given the weight of the seat though I'd definitely recommend getting a proper heavyweight solution made for them if you can find someone locally as the cost really wasn't prohibitive.
We bought the seats from the same seller linked above last summer and did a day trip up to York to collect and dealt with a really nice chap from the Yorkshire Steam Railway C5 TV show which was pretty cool. They all came with the plain blue headrest covers hiding the GWR branding which we've removed.
There is also a small tool you use that unclips the base seat cushion allowing far better cleaning of the base and surrounding areas (which predictably was FILTHY).
Oh and I'd definitely be pushing my luck with the other half getting cushions etc for them
I've requested to join the Facebook page linked above where it's no problem to post more pictures of the setup if accepted.
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yeah I get your experience with the fabricators wasn’t that good it is also another factor I have to account for as I don’t think some fabricators in London would have ever dealt with train seats! I never knew such a minor mistake could make such a big impact I’m going to hav tot be super careful when we get out fabricated although the final product looks quite great! Feel free to share photos here as well I’m always active here lol. Do let me know what the average price we would be looking at for fabrication as I have to account for that which I haven’t had time for and I’d love to talk on here more and hopefully you could guide me with my double seats and 
but I’ll surely give it a go!
Lack of rotation aside on this one, I genuinely can't think of many better chairs I've sat in for a few hours. 
Fittingly it of course doesn't work which was often the case when I would turn up for my train and find that reservations were voided because they couldn't be uploaded...
