Yes, all London mainline stations have an underground station as well.
Berkhamstead is too far out to be served by the tube. There is a mainline service to London Euston.
Euston tube station was created in 1968.
Euston Railway Station in London, England, was built originally in 1837. As the United Kingdom's sixth busiest railway station, Euston Station was rebuilt and expanded from 1962 to 1968.
The actual mainline station is all on one level (no steps or lifts). However, the underground station does not have lifts, but escalators instead.
The Trainline web-site says - take a tube train from Euston to Paddington, then a first Great Western service to Reading. Journey time Euston to Paddington is 28 minutes and Paddington to Reading is 25 minutes.
By tube (underground). Take the Northern line from Euston to Tottenham Court Road and change onto the Central line to Marble Arch.
Romford doesn't have a tube station but the mainline station has a frequent service to London Liverpool Street. The nearest tube station is Upminster.
The best option is to walk to Euston Square Station - which is well signposted and a two minute walk from Euston Station. Then, take the Hammersmith and City Line, Circle Line or Metropolitan Line to Liverpool Street.
There is no mainline station at Heathrow. You would have to take the underground into London Euston and get a train from there. You will have to change trains at least once and the journey from Euston should take between 5 and 6 hours.
4.7 miles by road.
Glasgow Central to London Euston, short walk to Euston Square tube station, then on the Hammersmith and City line to Hammersmith.
Euston most likely, due to the trains coming in and departures.