It appears the ratification of the UK Trade and Cooperation Agreement has simplified some of the uncertainty around this, and most EU/EEA visitors (with important exceptions for visitors from Iceland, Norway, Liechtenstein and Switzerland) to the UK can continue to use EHIC cards to cover the treatment that they did before Brexit (note that the transition period has ended and we are now nearly 6 months into "actual Brexit", although the COVID19 pandemic has limited travel and made it difficult to identify which rules have changed and which haven't sometimes).
To quote the relevant section of the webpage from the NHS, the UK's national health service:
Unplanned treatment on a temporary visit to England
If you're a visitor from an EU country and you fall ill or have a medical emergency during your temporary stay in England, you can continue to use your European Health Insurance Card (EHIC).
Your EHIC will cover you for treatment that becomes medically necessary during a visit to England, until you return to your country. It also covers you for the treatment of pre-existing medical conditions and for routine maternity care, providing the reason for your visit is not specifically to give birth or receive treatment.
Bear in mind that the healthcare system in England may be different from that in your home country. The EHIC might not cover everything you would expect to get for free back home.
The EHIC is not an alternative to travel insurance. It will not cover any private medical healthcare, being flown back home, or lost or stolen property. Therefore, it is important to have both an EHIC and a valid private travel insurance policy.
Given the "medically necessary" limitation in the passage above, your girlfriend may still wish to protect herself against unexpected healthcare costs which aren't covered (e.g. dental treatment) during her trip, but doesn't need coverage against typical accidents or emergencies, which are what usually push up premiums. Having said that, most insurers will require her to possess and use the EHIC where it does apply.