Just browsing through Decathlons website and a few decent buys noted on there.
See below
11spd 11-40 SLX cassette £29.99.
https://www.decathlon.co.uk/slx-m7000-11×40-bike-cassette-id_8392618.html

as above but in XT flavour £39.99.
https://www.decathlon.co.uk/11-speed-deore-xt-m8000-bike-cassette-11×40-id_8392616.html
Also availble in 11-42 for the same price.

And a cheap 10spd 11-40 cassette from Microshift £24.00.
https://www.decathlon.co.uk/10s-11×40-cassette-id_8378232.html.

Some cheap Sram NX shifters and cranks available too.

I bought the NX shifter and mech the other day.

And have a n XG1150 Cassette on the way, cheapest I could find.