I don't think the money gained is worth it. When Labour scrapped the assisted places scheme to put money into key stage 1, it evidently wasn't worth it as we have been repeatedly told many children still aren't leaving primary school with a decent level of ability.
I also think the timing is bad. The baby boom of the mid-late 2000s are still working their way through schools for the next 5 years so there is pressure in places in key stage 4 and 5. Projections I read yesterday are about 80000+ more 18 year olds for several years to come, than 10-15 years ago.
There is already a lot of support for disadvantaged school children mandated by the Office for Students, funding for which comes from university fees. And many educational charities in this field e.g. the Sutton Trust, the Brilliant Club, to name 2 of scores out there, national and local.
I for one will be sad to see the middle level independent schools go. Fee increases won't affect the really rich, so won't affect social mobility as the Eton types will continue.
I'd much rather see a high earner's tax, and more tax on big business. 2% into the pot from independent schools is not worth it.