I have lived near the Euston~Brum train line for 25+ years. I can get to Watford Junction for fast trains and the other stops for the slower ones. In the past, I have used it for going to trade shows at the NDC. Also to other points North.
For some years, I used to cross-check the equation:
- Time to reach terminal
- Cost to reach terminal (rail, coach, road+park)
- Cost of flight
- Return
For these in order of proximity to my dwelling:
LTN; LHR; STN; BHX; LGW.
After three or so years I stopped checking BHX because the cost/time NEVER worked in their favour. I have not checked recently but have no incentive to as it was not good before and I have so many airports to choose from.
By the way, IF there was to be such a thing as close LHR, then BHX might be of more interest as traipsing across to the Estury would be very time consuming - even if there were such a thing as Crossrail going to there from Euston.