I've been doing the LON - MAN route every 2 months for the last 4 years. Over the years, i've done LGW and LHR (both BA and BMI). In terms of reliability, LGW (Jet2 when they did the LGW route were best, but BA do a good job as long as the weather is ok). BA from LHR can be good or be heavily delayed! As for BMI, very much hit and miss! More often than not, the flight is late to arrive but occasionally i've flown with BMI and arrived in MAN over 30mins ahead of schedule!
After 4 years, i have discovered Virgin Trains! After the delays (especially the awful 20-30 min wait in domestic luggage arrivals at LHR), i have given up on air fares especially with the £20 extra tax!
If air is a must..... LGW anyday, the departure lounge at LGW is much nicer than LHR dometics, but like most of us on her, have to admit...... the government has forced us onto trains! How can you justify the luxury of the BA/BMI lounge when it cost significantly more than the train or car!