I have tried a dummy booking on their website and the price is replaced by miles. what they call "taxes, fees, charges" remains the same. Hence, you still save something like 40 gbp on the ticket. But anyway, maybe a misunderstanding somewhere...

It is not worth using miles for short haul flights these days, they are much more valuable for Eco or Business trips on long haul. Plus, they would have to give me miles for me to accept to use Heathrow or Gatwick....