You are talking about the cost of redeeming miles, yet the transaction you describe also involves buying some miles - this in part explains the bad value.

For random dates picked next year [outbound Wed 1st April in Economy, return Fri 17th April in Economy]:

- AVIOS (All) = 50,000 + £436.86
- AVIOS (Part) = 25,000 + £626.86

- CASH (Direct) = £830

So redeeming your miles saves you £400 vs. the cost of buying outright if you have enough miles.

If you don't, you're effectively having to buy miles - this is notoriously bad value.

Doesn't take away from the fact that using your miles for longhaul economy is not good because the taxes/charges that BA/UK Gov charge are disgraceful.