You should check out hobby king,you being in the uk you can get b grade li-po packs cheap and have your 48v and around 20ah for a couple hundred bucks.managed proper you can get 2-3 years out of a set and for sure get 40 km range or more.
https://hobbyking.com/en_us/multistar-high-capacity-6s-20000mah-multi-rotor-lipo-pack.html two of these and your good! these would be around 12lbs or 5kg the weight of one sla and take up the same amount of space as one sla maybe a slight bit more but still worth looking into!
You were right actually. It was well worth checking out the Hobby King packs made for remote control devices, "drones" and r/c cars and helicopters etc. The 44,4v nominal that you get when running four of their 11.1v in series is
not less real world voltage than a 48v Li-ion battery on most Ebikes, because of the high "C rating" these packs don't suffer from voltage sag when heavily loaded. On my little "250w" overvolted and 333w continuous the peak watt usage would be only barely into the figures where having a high C rating makes a big difference. On the popular 1-3kw motors that are popular in the USA the lithium polymer packs have significant advantages.
The rate of voltage drop as you use up the usable voltage of the battery is different too. The Li-poly has a higher voltage when it is "empty" so actually it's average voltage is about the same as the Li-ion which begins with a higher voltage and ends with lower.
But then there are some other considerations. If you are a beginner and you read the posts about living with these batteries it does come across that everyone is getting through multiple chargers and power supplies before they find one that lasts a while, but not forever. There seems to be no charging system that is guaranteed to work for everyone all the time. Even if you make a list of the choices that other builders finally settled on, there's someone else saying that didn't work for them. There's no consistency, no consensus to follow so you can just trust the wisdom of the crowd, buy once and have reliability.
The Li-po (lithium cobalt) recharge quickly but..
The batteries and chargers and power supplies should be placed in a fireplace during the charge process and must be monitored.
You have to carry a pair of server power supplies and a very large charger and the battery medic etc if you are going on a road trip. Not like the compact little chargers used for the common Li-ion "ebike" batteries.
So for me, for touring and fishing expeditions, I don't think that Li-poly is a practicable option.
I need to be able to carry a small charger or two in my panniers, with a campsite electric hook-up (or whatever you call them, RV mains plug and cable), and charge my battery inside my highly flammable tent while I sleep, or on my gasoline loaded bicycle.
But I'm glad I had a look at these. They do look nice and compact and good for high power "sport" ebiking nearer to home.