Me neither Michael - I'm just going on what the bathroom salesman told me!
Whether wall mounted mid bath taps need more pressure or something I'm not
sure.

I can assure you that if the taps with the largest commision on that he
wants to sell you aren't capable of low pressure operation, then you can buy
some from a retailer with a larger range.

Although there are indeed high pressure taps with thin passages suitable
only for mains pressure, there is no relationship between the manner of the
tap's attachment to bath or wall and this design "feature". The Victorians
used wall attached taps and they certainly didn't have combi boilers.

Christian.