Hi there,
A "standard" LHR-ORD-MCO will get you about 9916 base miles and the fare does seem to be about GBP 350 everywhere.
You can squeeze a bit more out of it by routing ORD-IAD-MCO for little extra cost.


If you're feeling nuts, then you can add cost and get quite a few more miles. Here's an example found on Expedia for GBP 423
LHR-IAD-LAX (Overnight) LAX-IAD-MCO
MCO-ORD-LHR
That'll give you 13968 miles.

There are a few more bits of evil you can wring out of this, eg for GBP 431:
LHR-MUC-IAD-LAX
LAX-IAD-MCO
MCO-CLT-ORD-IAD-LHR
That'll give you 15516 miles

Have fun!
Jason