so have I got this right, there are subsidies through tax benefits for highly profitable companies, that requires funding from the public purse, and increases current profit that they would have paid tax on but dont? How is that fair?
Setting a cost of doing business against taxable profits is not a subsidy, it is a legitimate tax deduction, a tax relief. This is common to all industries and throughout the world. Those profits that have been made are going to decrease as the decom costs are paid, so why shouldn't the taxes decrease as well?
The "funding from the public purse" is indirectly by way of lower tax payments by the companies, it is not the government paying directly for any of the decommissioning cost (AFAIK). And the tax relief doesn't mean the companies don't suffer any of the expense of the decom, they are still paying the net after tax amount of the decom cost.
Before any company would embark on developing an oil/gas field (and this applies to any other industry with a long-term project) they will look at the whole life costs and factor in any end of life costs and whether there would be tax relief on these. This relief was known about at the start of these projects.
Say I ran a successful caravan park, making good profit, then I had to dismantle said park as caravans became too old, can I offset these dismantling costs from the taxpayer, and enjoy all the profits I’ve made to that point? Isn’t it just part and parcel of a business that has a limited lifespan, like oil and gas extraction?
I expect you could, I don't know as I've never been involved with such a business and I'm not a tax expert. You are right, it is "part and parcel of a business" which is exactly why it's fair to have it as a tax relief.
If you thought you could develop the caravan park for £100k, would have a total of £200k of operating costs over 10 years, and total revenues of £500k, and knew you would pay 50% tax on the £200k profits, leaving you with after tax profits of £100k, would you still go ahead if you knew the dismantling costs would be £100k? No, but you might if you knew you would get tax relief on those costs.
I am more than happy to be put right and learn.
Also, taxes on oil companies in the UK are much higher than other industries. A higher CT rate of 30%, a supplementary rate of 10%, and an energy profits levy of 35%, a total of 75% versus 25% for most other companies. It used to be even higher than that for some oil fields, up to 83.25%.
And yes, I do work in the oil business and it saddens me, not that you are necessarily saying this, that some people think there are benefits to running down our oil industry (losing jobs and tax income) and instead importing energy, often from countries whose environmental and other standards are lower than ours, and whose production and transportation carbon footprint are very often (much) higher than ours.