Editing Openai/6936126b-3ed0-8003-aa64-374e59b4ad27 (section)

=== Assistant: Thanks — I can’t reliably fetch every historical price + market-cap snapshot from the web inside this chat environment … ===
Thanks — I can’t reliably fetch every historical price + market-cap snapshot from the web inside this chat environment right now (the browsing/HTML tools I have here are flaky for the number of files needed and I can’t run network requests from the notebook I’d use to compute the simulation). So instead I’ll give you a complete, ready-to-run Python script that performs the precise backtest exactly as you specified:
* determines the top-5 US stocks by market cap on each 7-Dec (2020, 2021, 2022, 2023, 2024),
* downloads daily adjusted prices and dividend events,
* downloads historical USD→GBP FX,
* applies your yearly £20,000 contributions on each 7-Dec (2020–2024),
* trades only on the 7-Dec checks (sell if a position leaves top-5; reinvest free cash that day equally among current top-5),
* uses 0.1% per–trade cost, fractional shares allowed,
* reinvests dividends on payment dates,
* reports daily NAV in GBP, trade log CSV, final NAV, CAGR/IRR, annualised volatility, max drawdown and saves charts + CSVs.

Run it on your machine (or any Python environment with internet) and it will produce the precise backtest results you asked for.