Arriving in terminal 2 going to 5 heathrow

Not sure if the "normal" flow you allow you to do this, but note the following items from the Heathrow shopping FAQ:

Can items be transferred between terminals?

If a product you would like to purchase is available at any terminal in Heathrow, we can arrange to have the product transferred to the terminal you are flying from, provided that the brand has a shop in your departure terminal so you can make the payment. Some brands offer a telephone payment service, in which case they can accept pre-payment and we can meet you with your purchase. Please enquire about this with our Shopping Services team.

Can I shop in other terminals not just the one I am departing from?

If you have enough time before your flight, it is possible to be transferred to another terminal to visit stores that are not available at the terminal you are flying from. To do this, an escorted transfer needs to be arranged in advance, which can be requested using our Terminal Shopping Transfer form.

Because of the staff costs involved in offering this service, we do ask that passengers commit to make purchases of £250 or more when using the service.

If I am being escorted to another terminal to shop, how soon must I arrive prior to my departure?

We recommend 3 - 3 1/2 hours before your departure time to complete the transfer process and allow you time to make your purchase.

Your best bet is probably to contact the Heathrow Shopping services team or the specific store you would like to buy from, they should be able to assist.