London to Brighton - Where to stay ?

Stay in Brighton.

Use the travel service that BHF offer to get you to London on the morning of the ride.

AirBnB is good in Brighton as it is both a tourist town and a university town. You should be able to get a room for a decent price.

The ride is quite easy with only three definite uphills - two quite short and the other Ditchling Beacon. The Beacon can be ridden but as mentioned earlier you are often forced to walk due to the shear numbers already doing so and taking up the road. If you are aiming to do a fst time you will need to be on a very early start (start times issued by BHF but you can ignore them). A later start time will see you take almost an hour just to get out of London due to the volume of cyclists (15000 official but probably another 6000 who just join in).

After the finish enjoy the seaside and all the activities and leave Brighton later in the day to avoid the traffic.

It's a fun event if not taken seriously.

If you're getting dropped off in London after staying in Brighton, you will be starting probably a little later. The ride is undulating, with the major climb Ditchling Beacon, taking around 7-10 mins of climbing, it has several false crests, just be patient and pace it up. once over the final crest it's all downhill to Brighton. 54 miles and 3500ft of Climbing. eat and drink regularly, take little rests if feeling tired in the legs about 3-5 mins

You could try www.warmshowers.org

Stay as close to Clapham Common as you can. AirBnB is probably your best bet as hotels will be silly money. Clapham Junction and Clapham High St train stations are both  short rides away from the start if you stay further away. Best not try to get the bike on the tube.

As mentioned, Ditchling is the only climb worth worrying about. Even if you can do it without walking, you may have no choice as many get off the bike without leaving room for others. It’ll be very busy there and at the start. Have patience and enjoy it.

I did it a few years ago.. there's a couple of drags and one hill (Ditchling) near the end.. however once over Ditchling, it's downhill all the way to the finish. 

Get off in an earlier group, take your time, enjoy the lunch stop and be wary of the traffic, as they can get a bit tetchy about 1000 bike riders slowing them down on their way to Waitrose.