Thanks Mike!
Yes, sorry, I did mean Surrey Quays.
Quite a few different routes and price points to consider, then!
I think, with luggage, the best option for me is probably the quickest route with the fewest changes (Slade Green > London Bridge > Gatwick Airport). Splitting the fare at East Croydon that’s £13.70 on contactless or £14.80 with two paper tickets. Both of these compare favourably with the £17.50 contactless fare not splitting at East Croydon.
On this occasion I think I prefer the paper tickets as it will enable me to split the fare at East Croydon without getting off the train.