I was looking at these a couple of weeks ago. Would that be Cash Isa or Stocks and Shares ISA or both? There are also different rates depending on how long you want to tie your investment up for too .........
A starting point could be to look at comparison websites such as Money Supermarket or Go Compare to get an idea of what's on offer at the moment:-
http://www.moneysupermarket.com/investments/isas/
http://www.gocompare.com/savings/guide/cash/
Products often seem to change monthly.
Good luck
