For myself, I only ever need a single roll of membrane, so that's £40, I already have the adhesive as I'll be using the same stuff to fix the tiles, no need for any of the band, simply cut a 200mm strip off the main sheet for internal corners. Domestic showers really only need 1m of tanking, especially over baths and in particular no more is needed unless using mosaic tiles. The OP didn't mention the polyester tape, that's £15 a roll, 2 required.
So to summarise, the OP could have popped a cherry on top of his eloquent reply, it's still not convnced me.
Remember, I have a choice as to what system I use, WP1 is gathering dust all round the country in shops, in fact, Topps stopped pushing it over 2 years ago, I needed to use some on a curved floor to wall joint in a wet room, two of my suppliers had WP1 in stock but it was out of date.
Just to add my two penneth, why are you using topps, Dont you have a CTD near you?
I assume you are Bathstyle from screwfix? if so i had noticed quite a long time ago that you have pops at people who dont agree with you or your methods.Me, well i dont care personally.
You have your ideas and work methods and thats great, other people have theres and thats great.
But having a dig at Richard is just pathetic mate.He has offered more help,advice than a hell of a lot of people on here, that also includes me.
Thats my opinion and i am entitled to it,same as you are entitled to yours
P.S
My local CTD dont have mounds of WP1 gathering dust, maybe Topps do as i dont know many tilers who use them to be honest with you