Aylesbury. Train into Marylebone in under an hour.
Whilst this is true the 1.5 hours door-to-door is a bit of a struggle unless you live next to either one of the train stations and work within a 5 min walk of Marylebone. Houses in Aylesbury have also gone up a lot over the past 5 years; if you find a 4 bed one for £200k, there will likely be a catch – although there are a lot of new builds with shared ownership options.
However, leaving early in the morning Tring is only a 15 minute drive and has trains that get into Euston in 40 minutes.
Don’t bother looking for a 4 bed in Tring, it would be well above £200k. You might in Milton Keynes or Luton though, both of which have stations and trains into London. MKC to Euston is around 35 mins I think and there is plenty of riding not far away, it’s probably the OP’s best bet.