For beet jelly you can either use water from boiling peeled beets, or you can simply use tough, overgrown beets for this.
There's no specific amount of water to beets, simply enough to cook the beets with extra water to spare. Top, tail, and peel beets. Slice into chunks or slices (plastic gloves are great for this job). Place in pot with water (no salt) and boil until beets are cooked. Slot out beets (and send to compost) and skim off any scum.
I make this recipe in the quantity given below as I've not found doubling or tripling works for me, but it might for others.
Beet Jelly
3 cups cooked beet liquid
1 pkg pectin plus 1/4 to 1/2 extra package if using less sugar
1 pkg raspberry jelly powder
Stir all 3 in tall pot (this can foam a lot later) and bring to boil, boil hard for 1 minute.
Add 4** cups granulated (white) sugar, stir well, bring to hard boil.
Remove from heat. Let sit for about 2 minutes and the foam will be easier to remove.
Jar immediately, seal, and if preferred, (which I do) waterbath to seal for about 10 minutes.
For whatever reason, sometimes the jelly doesn't jell immediately, but will jell in a few weeks. Sometimes it won't jell at all. I suspect either the pectin or the sugar was the cause. So I simply empty all the non-jelled bottles into one pot, guesstimate how much extra pectin to add (sometimes a box or 2 for a dozen or more jars), bring to hard boil for one minute, jar and seal. This usually does the trick.
** I find the 4 cups sugar a bit too sweet so I use only 3 cups sugar with extra pectin.
This is very tasty on toast or with peanut butter sandwiches in place of honey.
I've just remembered that package sizes differ. In North America, our pectin comes in 57 gram sized packages. Flavoured jelly comes in 85 gram packages.
Please adjust your sugar and liquid accordingly in the UK.