EHIC is the European Health Insurance Card covering certain types of healthcare in certain countries.

For UK residents, details can be found on https://www.nhs.uk/using-the-nhs/healthcare-abroad/apply-for-a-free-uk-global-health-insurance-card-ghic/

For UK residents, an existing EHIC card will remain valid until the expiry date on the card. These will be replaced by either a UK EHIC card or a UK GHIC card.

You should always take out adequate travel insurance for existing medical conditions when you travel, regardless of whether or not you hold a EHIC or GHIC card.