Have cycled London to Brighton BHF route quite a few times - both as part of the charity ride and solo. It’s an amazing route, but without the road closures can be quite busy roads. 

So I decided to check this route out - modified from Richmond park to pickup just after Carsholton. Some pros and some cons. 

Pros: 

- much quieter roads in much of the middle section, 

- lots of pretty countryside, with gravel sections taking you through nice places you wouldn’t otherwise see (but see below)

-farthing downs nice view (but have to go through coulsdon first)

- rejoins BHF route to take you over Ditchling beacon, which is a joy with a beautiful view as payback for the gruelling climb.

(Tip -  always feels tough after 50-odd miles. If you’re not a very fit / club rider make sure you’re fuelled and hydrated ahead of the climb and take it easy on the gentle lead up climb at the start - save your energy and pace yourself. )

Cons: 

- gravel/single track is a bit much for this ride to be honest - it’s just slightly longer than the BHF route overall, so adding those in slows you down and tires you a little. One small section after the motorway was just wide enough to get a road bike through with the spring vegetation! Another longish section was incredibly rough rocky gravel and tree routes. 
(I’m quite adventurous on my road bike with 28mm tyres and often find gravel to ride, but this small section was uncomfortable and quite hard work. And that’s with warm very dry conditions, no rain for well over a week - any recent rain and it wouldn’t be good on a road bike unless pretty grippy tyres. Also there’s so many nice country roads it seems a shame to waste them.
That said, if you want to really take your time and get off the beaten track, you may love it)

- it’s more undulating rather than the key climbs of the BHF, and actually more vertical climbing overall. Sadly you don’t get as many wide open views either. 
 

-The section through Coulsdon, just before Farthing Downs, is worse than the BHF. 

Took me about 5h moving and 5:30 total, starting near Richmond park and ending in hove (BHF route average about 4.5h moving)

I’m 50, reasonably fit (but not a club rider), road bike with 28mm tyres and some vibration damping. Am quite used to multi-day bikepacking and touring. 

Overall I was glad to find the route and try it, and I may use some sections of it in the future - the good bits were very good.