I'm just not sure if I can walk from Terminal 3 to Terminal 5 at Heathrow? Is this allowed? I keep seeing reference to the underground. All comments most welcome. Thanks.
I believe it is physically impossible actually to walk between these terminals but the road route is over 3 miles so you are unlikely to want to anyway.
If it is an airside transfer you want, use the buses between terminals via "transfers".
If you are landslide then you can use the Heathrow Express or London Underground free of charge. Collect a pass for the former and use contactless payment method for the latter to operate the barriers.
No is the answer. Distance between them far too long. Yes, hop on the Express to go from T3 to
T5. Use your tap debit or credit card to gain access.