Not far especially if you take the cable along the exterior wall and back in, bonding the water pipe.
I wouldn’t like to second guess Ohmes pricing structure, as it can be dowright extortionate at times. Material wise your looking at £1.25-£1.40 per meter of 10mm earth cablem £1.10 for a bonding strap and a couple of quid for a box of clips, with the rest labour charges.
I’d see what Ohme comes back with first or you could ring a local spark and see fit it in if they have a hour spare or on his way home. As a guide I’d charge £40-50 cash if I could fit it in a hour after work or if I had a spare hour hour during the day.
If your handy or know someone who is, its something you/they could potentially do with relative ease.