There are 3 types of ISA:
- Fixed rate/fixed term: no withdrawals
- Fixed rate/fixed term: withdrawal allowed with penalty (eg loss of several months interest)
- variable rate/open ended duration: you add/withdraw whenever you want
The fixed rate accounts offer higher interest rates because you're tying in your money for longer (eg 1, 2, 3 or more years). The longer the term, the higher the interest rate offered.
So it's a balance between finding a high rate but keeping your money (or at least some of it) easily accessible. That will depend on how much savings you have and what your spending plans are for the next few years (including some for emergencies).
Depending on your personal circumstances, you might want to consider putting some or all of your £6,000 in a fixed term account to get a higher rate of interest and then in April (in the new tax year) opening an easy access ISA.
That Coventry one matures in almost 3 years time and doesn't allow any withdrawals, although if you do need the money you can close the account entirely and they'll deduct 120 days' worth of interest.
I find that Nationwide and Kent Reliance Building Society are pretty consistent for offering high rates (at least for their fixed rates), so it's worth checking their rates as a barometer of good interest rates. Other institutions will offer eye-catching rates for a short period so it's worth keeping a look out for those too.
Last year Nationwide had a regular savings ISA (which is no longer available to new investors) with a decent interest rate so maybe look out in April to see if they offer another one. Other institutions might also launch regular savings ISAs in April.
Lastly, as interest rates are so low at the moment, if you are saving relatively small amounts then it might not be worth the hassle to chase the very highest rates. For example, if you had £1,000 for a year in an account paying 1.5% you'd receive £15 interest. In an account paying 1% you'd receive £10. That's a difference of only £5. Sadly you'd save more than that if you didn't buy a couple of magazines/bars of chocolate/glasses of wine during the year.
As a rule of thumb with today's interest rates, I'd say look for at least 1% on an easy access account, and aim for around 2% on a fixed term account.