Hammersmith and city line.
You would need to get a train from Waltham Cross station and head towards Liverpool Street station.From Liverpool Street station, head to the underground and look for the Hammersmith & City Line.Or get the 217 bus from Waltham Cross bus station and head to Turnpike Lane underground station. From here you can access the Piccadilly Line which will take you to Hammersmith on the Piccadilly Line.Roughly either of these journeys will take approximately 1 hour and 20 minutes, according to the Transport For London ( tfl ) website.
take the pically line to kings cross st parncas and take thameslink from there
The Piccadilly line is the only one that serves Heathrow, so take it to Kings Cross and then change onto the Northern Line (southbound) to London Bridge Station.
Take the Victoria Line direct from Euston to Victoria. Alternatively, take a cab - however, it will be considerably more expensive and probably won't be quicker.
Depends by what you mean by "around London". If you mean in London it's only Kings Cross. If you mean from "some miles around London", then add Stevenage (same line as Kings Cross).
Kings Cross
Take the Piccadilly underground (tube) line from Heathrow to Kings Cross and get the overground train to Ely from there. The whole journey should take about two hours thirty minutes.
Walk to Euston Square which is around two minutes away and well signposted and take the Hammersmith and City Line or Circle Line to Paddington.
About 1 hour
Trains don't go from Kings Cross to Manchester. They leave from London Euston which is a 10 minute walk from Kings Cross. The journey from London Euston to Manchester Piccadilly takes around 2 hours 10 minutes.
London, Euston to Kings Cross, St. Pancras is just one stop on the Victoria line and takes about 3 minutes by tube.However, as they are so close it may be quicker to walk direct to St. Pancras, just follow the direction signs.
The London Eurostar terminal is at St Pancras International station which is adjacent to Kings Cross station, so whatever it takes to cross the road and buy your ticket.