Oil and gas production, in the North Sea, peaked around the year 2000 and has fallen by more than 2/3 since (source: https://obr.uk/box/the-rise-and-fall-of-oil-and-gas-revenues/). There’s a reason for this. The easily found, cheaply exploited, large deposits have been used.
What’s left is smaller, harder to reach and, hence, expensive fields. We could indeed exploit more of our own oil and gas but it wouldn’t be cheap. We’d have to pay more than we pay on the open market.