Credit unions don't pay interest on normal savings, per se. Instead they pay dividends based upon the return that the credit union has obtained for investing the money of its savers. (i.e. it's somewhat akin to investing in shares but without the risks):
http://www.policecu.co.uk/savings/rewarder-savings/
Their cash ISAs also don't offer any guaranteed figures but the PCU has a current projection of at least 2.5% (tax free). (I've written 'current' but that figure is actually derived from a web page which is clearly in need of updating):
http://www.policecu.co.uk/savings/cash-isa/
With the Bank of England base rate currently at only 0.5%, you won't find any really good rates of return for any investment (unless you're prepared to risk big losses of course). The PCU might be as good as anywhere but if you'd prefer to have actual figures (rather than just projections) you might like to look at the alternatives here:
http://www.moneysavingexpert.com/savings/savings-accounts-best-interest
Chris