Nothing stopping you manually lifting the chain onto the right ring as you need to if you can make the front mech/shifter combination work. So you could ride most of it in the big ring, and pop the chain on the granny for the beacon.
Enjoy, like many on here I did this ride countless times from the 80s onwards. Great memories, nice way to meet cycling ladies too at the pit stops. Often stayed the night in Brighton, partied then came home the next day on the train when it was less packed. I remember seeing my £££ bike abandoned by the shuttle (which went separately) along with lots of others in Islington iirc. Just stacked up for anybody to steal. Never ran so fas to my P&J.
Really enjoyed it, it was quite warm though. Especially going up ditchling beacon, which was a struggle . I’m blaming the heat. Just about managed to do it without stopping and did the whole thing in 4 hours ish. Thanks for all the advice!