Not used--but terminal 3 @ LHR (heathrow) has designer stores. If your airline departs there, nice place to save VAT, etc.
Unfortunately, I don't know if mulberry has a store there.
So, this advice may not help you.
For myself, I choose this route & use my London time to experience the city.
Yes all Heathrow Terminals have Mulberry and you can shop online, reserve and collect when you get there saving 20% is a great discount!Is Norway in the EU? If not you can do tax free shopping in the Uk, you need a form to be stamped by the store and then when you go to the airport you take your form and purchases to HMRC and you can reclaim the VAT. If you google Tax free shopping you will find lots on information.