I'm on about 60-70ft lines, as with all kites short is nice for control and feel! some fly on shorter than this for tricking, some go longer - nice to learn on. I'm still learning, find it a good length to work with and most STACK competitive flyers on around this distance.
I try to fly on 100 ft, 90lb or 50lb normally, this gives you more space and time to work in i have shorter 60 ft @ 50lb for light wind, and a more flicky/tricky style of flying. i also have a120ft set , this is the length teams fly on.
Erm....Team lines 120ft....ideal for when your learning as well, gives you a lot more time to react than shorter ones, you will have a greater area in the sky (wind window,an arc from right to left or left to right ..lol)......your reaction time with shorter lines will have to be very quick, not really what you want when learning.....
Longer is more fun if you're not into tricks and tracking the kite on shorter than 75ft makes you feel like you're going to tip over like I do. I prefer 85 or 100ft but crap winds often make those lengths feel like I'm flying through different time zones.
I much prefer Shanti Speed lines because they don't grip and groan like LPG. Nothing I've noticed flying both types offsets the g & g'ing of LPG.
LPG were sold to me as being stiffer and less prone to tangles. I didn't find that as true. Paying attention works best for less tangles. Most tangles I've had turned out to be false tangles I turned into real one's while setting up. John Baressi did a You Tube Rev series that includes a set up vid that made a difference for me.
From the U.S. Shanti Speed Line is available through A Wind of Change and Into the Wind. Both are reasonable and reliable with somewhat different offers on bulk line. Think sourcing from U.S. shouldn't be a problem. Don't know about U.K. availability.
Rambling a bit I guess because my flying field has been closed about 6+ weeks starting right as the daily thunder storms of monsoon season ended and not flying presents an existential crisis.