Yeovil or Castle Cary

Advance tickets will be the cheapest, if you're happy to commit to specific trains a couple of weeks in advance.

Looking at your days next week, there are no particularly worthwhile Advances available, suitable tickets would be:
Yeovil Junction - Waterloo Anytime Single, £47.20
Waterloo - Yeovil Junction £29.85 Off Peak Single (NSE) or £25.50 1st Advance on the 1620.

Castle Cary - London Off Peak Return £74.00, valid on the 0744, arriving Paddington 1039 (change at Bath), valid on all but the 1736 return service (direct)
Advances for these trains (or most suitable trains for you) come to ~£50 at the moment - so you don't need to buy far in advance on this route.

The Network Railcard easement is still in place, allowing travel on the 0826 from Exeter St Davids - but I'm not sure if Yeovil is included, the wording is unclear but it seems that the Railcard is allowed for boarding as far as Feniton, but not east of there. In any case, that train gets to Waterloo at 1149, too late for you.

 

I couldn't help with the unfeasibly complicated world of fares, but to get to London by 1100 from YJ you'd have to leave no later than 0829, arriving 1049. From castle Cary there's a 0727, arriving Paddington 0921. The next one after that'd be 0941, arriving 1124.

 

What's wrong with the £51.00 Off Peak Return Yeovil Junction - London Waterloo (route: Salisbury)? Valid for arrival into Waterloo after 1000, (0750 YVJ-WAT 1019) and valid for return any time after 0900, so fine for a 5pm-ish departure from Waterloo.

 

Obviously I don't know exactly which village you're in, but midway between Castle Cary and Yeovil sounds as though your most convenient station might actually be Templecombe. Even if it is no nearer than Yeovil Junction, it will be a few bob cheaper to go from there.

 

I'll get me coat...