All 28 UK money calculators are free to embed on any website or blog. Pick one, copy the code, paste it where you want it. No sign-up, no key, no cookies for your visitors, and the figures stay current because they are served from here.
This is exactly what your readers will see, already in embed form.
Keep the credit line. The snippet includes a small "Free calculator by The Base Rate" link under the frame, and the embed itself carries an attribution bar. That link is how a free tool stays free. Beyond that: use it commercially, use it on as many pages as you like, no permission needed.
Not financial advice. All calculators are illustrative and educational. Figures are sourced from gov.uk, HMRC, the Bank of England and the ONS and re-verified when they move, but your readers should always confirm against the primary source before acting on a number.