I must admit I did wonder about the well tank, or even LSWR 02. But the former, although pretty, is very restricted geographically and the latter just has too many variants if they were to do done to suit anybody.
The R1 wouldn't be great choice either as they were Eastern section only, I think (I'd need to check). A 'P' is a thought as they even ended up working from Brighton in the forties and fifties.
I've just had a thought it might be an E1; widespread and long lasting, but surely just far too boring?
In truth I don't think I can make a convincing case for a Southern tank loco (other than a Terrier) as they tended to be pre-grouping designs and restricted in their travels. So whatever they produce will be a surprise for me and once I know what it is I'll make a decision about whether I'll buy one.
Steph