You can certainly visit the terminals, apart from the domestic wing of T1, but you won't get lounge admittance in normal circumstances. There are no boarding pass checks from airside, except when leaving domestic T5 and domestic T1.

However if you are doing this, your sensible chance of doing this is directly after your arrival into T5, so going immediately to T1 (etc) on the bus service. Once airside in T5 you cannot easily get to the buses. It also gets complex leaving from airside T1 and T3 to return to T5. It can be done, but you'd need to know where to turn. HAL provide a shopping service escort for this but I think they are looking for a spend in the hundreds of pounds to facilitate it.

All in all, not worth it.