GPT-5.3-Codex vs GPT-5.5for everyday maths

No one can give an exact number: ISA returns aren’t fixed (markets/interest rates move), and provider fees can differ.  
A reasonable nominal estimate is to assume **2%–8% annual growth** for 10 years:  
**£10,000 → about £12,200 to £21,600** (before fees).  
For a midpoint assumption of **5% yearly growth**, it would be about **£16,300**.

No exact figure exists because ISA returns depend on future interest rates, investment performance, fees, inflation and tax-rule changes, none of which are knowable in advance.

If it’s a diversified stocks-and-shares ISA and you assume **3%–7% annual growth after fees**, £10,000 in 10 years would be about **£13,400–£19,700**. A cash ISA would likely have a narrower but usually lower range, depending on future rates.