you could divide by 1.2 for airport price ( so if your shawl is 760GBP, the airport price would be around 633 GBP, which is a savings of 16-17%). If you buy in Central London, the tax refund would be around 10-13%. But with H, just grab it be it in Central London or Airport.
I always think shop at airport is better (cheaper and no hassle) until my recent regret. I skipped a beautiful GP in Stockholm boutique, thinking that GP is easy to score in Paris airport. Big mistake

So, when you see the things you like, just grab it.