englishkev
Had the same problem in Heathrow's terminal 5 last month again in WH Smiths.I only wanted to buy a bottle of coke and a newspaper.Yet at Manchester on the way out it wasn't required
My guess is that part of this is due to those terminals where inbound and outbound, domestic and international pax mingle. Depending on your destination and status the VAT rate may be different?
For several decades, terminals have been designed to ensure that In/Out and Domestic/Int pax could not mix, so as to keep contraband and nefarious materials out of circulation. That is, lax security at one location enabling materials to be delivered to another and handed over before the pax went land side.
As I understand it, for LHR T5, all of this was thrown away - just so that they could have ONE set of shops. This enables them to charge more rental to the shops as the shops have a larger pool of pax.
I sit to be corrected on these points.