Are those prices just for labour or including the fittings?
The fittings can vary substantially. Actually fitting it all and doing the work should be fairly similar whichever workmen you use and although there will always be some variation, that’s a lot.
If it involves putting in plumbing etc I’d assume the £3.5k quote is just for the work. £8.5k for just the work seems a lot unless there are drastic plumbing and building aspects.but that would be the same for the other workman too.
Ask for a breakdown of what the quotes include. Is it possible there could be other costs for the £3.5k one.
The smaller the en-suite, the more carefully you have to choose fittings and think about how it all holds together. What are your room dimensions?
If you’re getting a bath, try not to buy until you’ve sat in it as the shape and comfort for you personally varies and it’s hard to tell from online or catalogue pics.
If it’s a shower room, consider if you have a cubicle with doors fully enclosed (quadrant or square/rectangle) or a walk-in with no door.
Check the door opening...often opens into bedroom to give more space and ensure any rainout or towel rail won’t be where your bare bum might touch it when getting out of shower.
Consider storage for spare loo rolls, cleaning products and the odd towel. Do you want an inbuilt niche in the shower for containing products you use or will you have one of those metal rack things?
Will all the pipe work be chased into the wall or will there some which is on view or boxed in?
Often people need a small basin in an en-suite but don’t go too small or the water will splash crazily when you put the tap on.
Key thing is to be clear what your quotes include and don’t include and to see a breakdown.