MG sold 2,300 cars this year (Jan-Aug) and 3,152 in all of 2015 in the UK. In Thailand, the corresponding numbers are 5,018 (Jan-Aug) and 3,779 (2015)*.
SAIC's Hemaraj/Rayong plant currently has a capacity of 50,000 per year and the auto major has declared its intentions of making it a global hub for RHD cars, and of course there is little stopping it from making LHD cars there too. Note, this is in addition to what it can source from plant(s) in its homeland.
A second plant is under construction at the above Rayong location with an additional capacity of 100,000 units p.a. Most of this new capacity is intended for future Maxus-brand pickups however.
Not to forget that Thailand is a signatory to numerous Free Trade Agreements.
Within the conference rooms of SAIC, it seems there is scarcely any argument therefore in favor of continuing production (i.e. assembly) at Longbridge.
*numbers include CBUs from China