There are regular services from London Marylebone that get in at a decent time, there are six trains between 18.00 and 19.00, taking between 30 and 40 minutes. For example, the 18.36 gets to High Wycombe at 19.07. For the bus you need the 17.21.
However, not many after the game. There's a chance on the bus, or you jog very quickly, and then the referee is kind with any stoppage time, of the 22.06 from High Wycombe, back in London Marylebone at 22.55.
More likely, and you take a slow walk back, that will mean a very late return to Southend catching the 22.59 from High Wycombe, back in London Marylebone at 23.33.
The last train back to London is the 23.25, in at 00.04.
There is still an underground service, Marylebone is on the Bakerloo line then change to the Circle, and you can make the 00.15 from Liverpool Street.
The first Fenchurch Street service you could probably make is the 00.33.
To book go here:
www.chilternrailways.co.uk
This is a good half hour walk to the station so you may wish to grab a cab which costs around a fiver or get the football special bus, No. 501, that runs from the station to the ground at 18.40 for midweek games for about £2.50 return. The bus goes back to the station 10 minutes after the final whistle.
Here's a map to Adams Park, many thanks to www.wasps.co.uk:
