Are you connecting between two BA flights?
If so, upon arriving at T5, take the transfer bus from either T5A or T5B to Terminal 3. Scan your T3 departing boarding pass at FCC Security. Make sure you do not scan your T5 boarding pass. Clear Security and go shopping.
When you are done, follow the signs for Arrivals which start appearing near the gate 21+ pier if I am not mistaken. You will take the bus back to Terminal 5 where you will scan your T5 onward boarding pass (this should be the first time you scan it). Go to the lounge and call BA to refund your T3 departure.
To be comfortable you should leave T3 at the latest 2 hours before your flight from T5.