if you don’t return your Tax Free Form within 21 days,

a charge plus a debit fee of 15% of the refund amount will be made to your card.
なのです。

uk.westfield.com
London
SERVICES
Tax Free Shopping
the Ice Bureau will give you an immediate tax refund (conditions apply)
というに、その場で還付があるので、それを貰った上で、使うことは可能か否かを
聞いているはずです。

しかし、
You may then be able to claim a cash tax refund immediately at ICE Currency Exchange at Westfield London if your receipt has the Global Blue, Premier Tax Free or Tax Free Worldwide logo (although a small charge may be applied for this service). You must remember to keep your receipts as you will need to present the goods, receipts and completed documentation to border officials upon leaving the EU.
とある上に、

ほぼ同じ内容が、
TOURISM
TOURIST INFORMATION
Tax Free Shopping
Find More
詳細が、
Download Guide

HOW DO I CLAIM MY VAT BACK?
It just takes three simple steps

CASH IT!
Take your completed tax refund form to the ICE International
Currency Exchange at Westfield London for an instant refund. At Westfield
London you’ll find ICE on Westfield Parade. At Westfield Stratford City you
can pick up a refund form from Travelex on the Lower Ground Floor. Please
note that no refunds are given on site at Westfield Stratford City

これから先が問題で、
STAMP IT!
When you are heading home and leaving the EU, show your
purchases, receipts and passport to customs, and ask that your Tax Free
Form be stamped. Finally, post the stamped Tax Free Form to Global Blue
in the envelope provided. (Please note that if you don’t return your Tax
Free Form within 21 days, a charge plus a debit fee of 15% of the refund
amount will be made to your card.) At Westfield Stratford City you’ll find
Travelex on the Lower Ground Floor. Check the multi-lingual Mall Guides
for exact locations, or simply seek the assistance of a member of our
Concierge team.

IMPORTANT:
Keep your receipts safe. You need to show your receipts, goods, passport and
completed documentation to get your refund.

とある以上、不正はできないはず。