There is one major supplier of L1011 spares in the US, they have pretty much all the new parts and those taken off retiring Tristar. In other words the RAF has to deal with a monopoly who can charge pretty much what they want for certificated parts. They know that the L1011 is stuck in service for a few more years and the RAF can't shop around for spares too much.
Sounds to me the RAF didn't like the sound of the price for a replacement landing gear is casting around in a desperate attempt to find an alternative.