UK oil production peaked in 1999, at a time where the oil price ran in the $10-20/barrel range. At that time, total production of 3m barrels per day (bpd) meant 1m were destined for export. At a $15 average price point, for the UK this meant total income of:
1mbpd * $15/barrel * 365 days = $5.5bn in income contribution to the British economy.
However, this doesn't factor in production cost, which is generally said to be running in the $9/barrel, when you factor in exploration, extraction, and general production costs (for the record, these have in the meantime increased to around $22/barrel for North Sea oil):
1mbpd * $(15-9)/barrel * 365 days = $2.2bn in post-production cost income contribution to the British economy. The bottom line is that the British economy via the public and private sector have overall had income emanating from the oil sector.
However, as the UK is now well past peak, imports will necessarily rise, and just at a time when oil price is spiking due to a rising demand/supply imbalance.
Last summer, BP released the current status on localized demand and supply, and it shows the demand of the United Kingdom to be gradually reduced 3-5% on an annual basis, no doubt the direct result of higher prices, which puts a limit on marginal demand. However, this fails to keep up with the decline of supply, which runs in the 6-10% bracket (General North Sea decline is around 9% annually, with some estimates all the way up to 17%!).
In 2009, the demand and supply figures for the UK were 1.611mbpd and 1.448mbpd respectively, which essentially means the need for imports is the differential (1.611-1.448=) 163kbpd. In monetary terms, this means with a current Brent price of $120/barrel, the amount of money leaving the British economy on an annual basis amounts to:
163,000 * 365 * $120 = $7.1bn.
This is not an issue at present time, but if we consider demand/supply decline rates of 4 and 8% respectively, this is the equation to consider (The equations are admittedly simplified, but the errors introduced will not be statistically significant):
fnc(year,avg_price) = ((1.611*((1-0.04)^year)) - (1.448*((1-0.08)^year))) * avg_price * 365
Assuming the price stays static at $120/barrel (which is highly unlikely), these are the figures we get (annual, and cumulative):
2010 - $9.3bn, $16.5bn
2012 - $13.0bn, $40.9bn
2014 - $15.7bn, $71.2bn
2016 - $17.6bn, $105.6bn
2018 - $18.9bn, $142.8bn
However, with a 10% annual oil price growth, this is the equation to consider:
fnc(year,avg_price,avg_price_growth) = ((1.611*((1-0.04)^year)) - (1.448*((1-0.08)^year))) * (avg_price*((1+avg_price_growth)^year)) * 365
And here are the net results:
2010 - $10.3bn, $17.5bn
2012 - $17.4bn, $48.6bn
2014 - $25.3bn, $95.1bn
2016 - $34.4bn, $159.bn
2018 - $44.6bn, $243.2bn
The bottom line is that what used to amount to a gentle financial aid to the British economy, is rapidly turning into a huge drain, which will leave the British economy struggling, increase the pressure on the welfare state and hence increase the deficit. And this is not considering gas imports, which are rapidly growing as well.