We are traveling by air from Toronto to Heathrow and then on directly to Glasgow using BritRail passes. We would appreciate suggestions on how best to travel economically from Heathrow to Euston station (or alternative rail departure points to Glasgow, if any) with a minimum of stair climbs. There seem to be many alternatives but we haven't found enough information to evaluate them. Suggestions would b appreciated. Also, how much time should we expect to spend in customs and immigration? to get from customs and immigration to the recommended train, tube or bus? to get from Heathrow to Euston or alternate?