Go on money saving expert website, find a highest paying savings account. Fixed term accounts will always pay higher rate, so if you know you won't need all that money for a year or two, I'd look at those first.
Depending if you're basic rate tax payer, or a a higher one, divide 1000 (if you're basic rate) or 500 (if you're higher rate) by interest rate in decimal points (so 5% will become 0.05) - this will give you an amount you can save next tax year without going over the tax threshold.
Say the best rate today is 5.5%, and you're basic rate. You divide 1000 by 0.055, that gives you £18181.81 - this is the amount you can safely save without going over tax threshold. Personally, I'd round it down a bit, so put £18k into fixed savings. Then put the rest into ISA, but again, check the same website for best rates.
One thing to be aware of - if you go for 2/3/5 year fix, you have to divide that number by number of years, so in my example above, if it's 2 year fixed account, you can only save £9k before you pay tax in 2 years time. It's because you can't carry over unused savings allowance, it's applicable in the year you've received your interest payment.
Another option would be to stagger 1 Yr and 2 Yr fixes, so say put £18k into 1 Yr, and further £9k into 2 Yr, but you have to be very good with understanding/knowing how much interest you're earning each tax year to jot go over threshold.
If you don't want the faff of moving things around too much, then do the calculation, stick whatever you come up with in 1 Yr fix, and the rest into ISA.
Apologies if this is too confusing! Happy to work with actual numbers if you found the accounts you wanted to go for.