Upstairs landing light switch doesn't fit between the architrave. (See photo)
Options are fit architrave switch, although once flush mounted might be hard to operate or make a wooden square for a standard light switch to flush mounted to. Or move the switch, although it's current position makes the most sense.
The only other logical place has a socket directly below so would be difficult to route the 2 way switch cables to the downstairs switch
You did ask for ideas so, here goes.
While I am not sure of the dimensions of UK Wall-Plates, I do know that they are wider than US and Australian Wall-Plates, which have a nominal width starting at 70 mm - but some plates with covers may be 75 mm.
Because of this, you could use an Australian one or two Gang wall plate (See
https://www.ebay.com.au/itm/SC2000-...dDelivery!3129!AU!-1:rk:1:pf:1&frcectupt=true)
While I am unsure of the legality of using these switches in the UK, the same mechanisms are/were sold by Schneider Electric in the UK.
It is not just the switch that is causing the problem but the width of the back-box also.
Of course you would need to replace the Back-Box with one of appropriate size, which you should be able to obtain from the USA (e.g via amazon.com)
(About the only type of back-box obtainable in Australia is one like this [
https://www.bunnings.com.au/hpm-standard-wall-junction-box-with-sliding-nuts_p7053636] for masonry walls - since, elsewhere, back-boxes are not used in Australia/NZ)
If you just replaced the back-box with one of US/Australian size, you could make an adapter plate out of plastic to suit the mountings in that (or use a US or Australian "Blank" plate of 70 mm x 115 mm), and cut an opening in it to mount a UK Architrave switch - or a double UK architrave switch. (The mounting centers for the screws in US/Australian wall plates are 97 mm apart.)