If you live in London, and you have all this coast near you, why go all the way to Blackpool?
It is a horrible drive up the M40 then the M6 between Birmingham and Blackpool.
Bournmouth does have a reputation of being an old peoples place, but I gather there is also quite a good nightlife there.
The place to go is probably defined by which side of London you live.
If you are on the East why not Southend?
If you are on the South then you have a lot of choices: Hastings, Brighton, Bournmouth. Portsmouth.
Having been to most of those places I would still say Brighton is the best (not a great beach though, lots of stones).
But Brighton has a great wide prom, great pier, lots of great shops and bars, and the "lanes" an area of small antique and other shops which is really interesting.
Portsmouth is a large city, but also has a seaside feel to it so you get the benefits of being in a large city (pubs etc) but also good seaside facilities.
Lots of history in Portsmouth as well, making it an interesting place.