Mastercard Black travel Insurance in UK

There are no restrictions to entering the uk without travel or medical insurance.

The uk health system will treat any emergency without question, but US national are not entitled to full free treatment (unlike eu citizens, with which there is a reciprocal agreement), so the hospital, or gp would may be seeking payment for the services given, especially if you are admitted to hospital. The nhs website says

If you are taken to A&E (accident and emergency department), a minor injuries unit or walk-in centre for emergency treatment then this is free of charge. However, if you are admitted to hospital for any other emergency treatment a charge may be incurred. You should not go to A&E for non-emergencies, but see a GP instead.

But treatment for emergencies would not be denied just because you don't have insurance, but ongoing would be charged

Having said that, because the uk has a national health service any treatment would be given at cost, there is no profit motive. This doesn't mean that it would be cheap however.

I would say that for any international travel good insurance is important, especially as things like medevac back to your home country following and accident would be expensive.

MasterCard black insurance should be sufficient

http://www.mastercard.com/tt/personal/es/findacard/CreditCard/black_card.html

But remember most travel insurance won't cover an existing condition, so check with them for any exclusions