There is something odd going on here. G-BOAC is correct in saying that only fare classes Y, B and H are upgradeable...... Well that WAS the case.....
And doesn't seem to apply to round trip flights with one leg upgraded.....
For example, LGW - MRS R/trip for a weekend in the Autumn....travelling in CE pricing up at £373.00 PLUS 9,000 BA MILES. OUCH!!!
However, the same flights booked OUTbound in ET and RETURN in CE are pricing up at....£75.30 plus 4,500 miles. (INCLUDING ALL TAXES.... BARGAIN!!!!)
If you book the same flights OUTBOUND in CE and RETURN in ET... pricing up at £79.20 plus 4,500 miles, again INCLUDING all taxes.
It works for other routes as well (tried KRK and LIS)
Not sure if this is just a glitch, but thanks Jimbosmith300!