Is it only mammals you are interested in? There are several areas within the London area that are great for birds, and also offer a chance of seeing the odd mammal as well. One such area is the London Wetland Centre in Barnes (www.wwt.org.uk/visit/wetlandcentre). This site can be reached via public transport and is home to Water Vole, Red Fox and several bat species.

Rainham Marshes can also be accessed by train although is at the other side of London to Heathrow. It’s brilliant for birds and Grass Snakes, but also good for Water Voles and several of the more common mammals.

I’m not really familiar with London as a mammal watching location although I know there are several parks around the fringes which can be good for deer. For example Bushy Park in south-west London is home to conspicuous herds of feral Red and Fallow Deer although I’m not sure if you can get there on public transport.

Grey Squirrel is easy to see in any London park although I suspect they are not top of your mammal list. Hedgehogs are common in urban areas, but are mostly nocturnal and not easy to see on purpose unless you know of a garden/yard they visit regularly. All my hedgehog sightings are accidental.

Sorry I can’t be more help but I’m not overly familiar with London, especially the Heathrow area. Hopefully a more knowledgeable person will be able to assist.