Heathrow is a pretty spotter-friendly airport. I’ve been quite a few times now and never been moved on by the police.

If the aircraft are on the 27s, then a switchover happens at around 3:00pm. Both 27L and 27R will continue to be used, but takeoffs and landings swap around. If they are using the 9s, 9L will be used for arrivals and 9R for departures (with no changeover).

If you are arriving at Hatton Cross, then the airport is your oyster, with no shortage of buses serving the Hatton Cross Bus Station.