Best 18 Month Fixed Rate Cash ISAs in October 2026

Aldermore's 15 Month Fixed Rate Cash ISA pays 4.85%, which sits above the cash ISA market average but still 0.40% behind the category leader at 5.25%, so it is a solid mid-table pick rather than the best available. With CPI at 3.1% as of Aug 2026, the rate delivers a real return of 1.75% above inflation, which is a reasonable outcome for cautious savers.

The main catch is the £20,000 cap on deposits and the fact that your money is locked away for 15 months with no access. Transfers in are accepted, which makes it worth considering if you want to consolidate existing ISA balances.

Paragon's 15 Month Fixed Rate Cash ISA at 4.75% sits above the Cash ISA market average, though it trails the category leader by 0.50%, who pays 5.25%. At 1.65% above August 2026 CPI of 3.1%, it does at least beat inflation in real terms.

The account accepts transfers in and works as a flexible ISA, which is useful.

The genuine catch is the early exit penalty: 120 days' interest, meaning closure in the first four months could return less than you deposited. Aldermore offers a comparable 15 Month Fixed Rate Cash ISA at 4.85%.

Secure Trust Bank's 18 Month Fixed Rate Cash ISA pays 4.70%, which sits above the cash ISA market average but is 0.55% behind the category leader at 5.25%, so it is a decent but not outstanding pick.

That said, 4.70% does beat August 2026 CPI of 3.1% by 1.60%, meaning your money grows in real terms. The account accepts transfers in and needs just £1,000 to open, making it accessible for most savers looking to shelter a lump sum from tax.

The main catch is the early access charge of 120 days' interest if you need your money before 24 May 2028.