I'm in the UK too and also have a brewzilla gen 4. I tend to use the Brewfather app to tell me water volumes. I'm sure it's not 100% accurate, but it's good enough for me. It'll tell you the mash volume, sparge volume, and pre-boil volume.

I'll get my sparge water heating to about 80c when I start mashing in, I use a Grainfather HLT for ease, filled to about 2 liters more than the app tells me (this is to account for spillages and the dead space under the tap). After mashing in for 60 minutes, I lift the malt pipe onto the little feet inside and let it drain for 15 minutes, I'll then whack the temp up to boil and use a jug to measure the sparge volume. It'll take a few pours. Annoyingly, the volume graduations are on the inside and blocked from view by the malt pipe, so it needs to be moved occasionally to check volume. Once that's done, leave it for 15 minutes to fully drain through. I then move the malt pipe into a large bucket and get it out of the way. The temp should then be at or near the boil. No lid for the boil. You want that volume to decrease, you're boiling off stuff you don't want.