Yes.
Or at least, if you can do that distance on any other bike, you can do it on a Brompton. I have.
(Edit. Ah, it's "The" London to Brighton as in a specific ride. Have to say I haven't done that. Just "a" London to Brighton)
ln the FAQs:
6. Are there any requirements as to the type of bicycle or carriage I can use?
Participants don't need a specialist bike for the event, but it is recommended that they use a bike with gears and ensure that everything in working order and road worthy before the event.
It is essential that participants check: gears, brakes, the frame and wheels. If the participant isn't confident checking these then they should take their bike to a bike shop.
As the Bike Ride is on roads, a road bike is the most efficient to use, however it's not imperative. If a mountain bike is used, the rider will be better off replacing the knobbly off-road tyres with 'slick' tyres. This will reduce the friction and therefore make the job easier. The main thing is to check the bike is roadworthy before setting off and remember a helmet - this will reduce any head injuries in case of an accident.
Be proud of your Brompton. Many of the other riders will be envious.
I rode up some big hills on my route ( including white down lane to Ranmore Common) the biggest issue was that the wheels on the Brompton aren’t very big, and they weren’t in very good condition at the end of the Ride.The only challenges a Brompton is likely to give for longer rides are that you may have a relatively small gearing range more suitable for city riding (depends which model you have) and the riding position can get a bit monotonous. I tend to get aching shoulders after a few hours on the Brompton. If it's a windy day you can get blown around a bit, especially if you are a large target like I am.
On the gearing front, I doubt you'd have a problem. I think the BHF route goes via Turners Hill and up Ditchling Beacon. Apart from that there's not much in the way of hills (although it's not entirely flat) and Turners isn't much of a hill really - it's only steep for a very short section by the antiques shop near the top. Depending on your gearing and fitness you may have to walk up Ditchling Beacon, but from what I've read I think the road can be a bit blocked with people walking anyway.
I've also read that bikes are banned from trains on the day of the BHF ride but I guess you can get a Brommie home on the train! (You'd need to confirm that)
I signed up to the event and have a Brompton.
I mentioned it to somebody and ended up feeling quite silly due to what he said.
The bike is more than capable. The biggest thing that would make the journey difficult would be a lack of fitness.
If you don’t mind my asking... What would your average weekly mileage be?