You are a dual US/UK citizen. You are subject to US tax law in the US, and UK tax law in the UK. Under US tax law, the recipient of alimony pays tax. Under UK tax law, the payor of alimony pays tax. This dichotomy means that you could suffer US tax on the pension if it is deemed alimony. Pensions are usually taxed in the country of residence, but there are a number of tax breaks on pensions under UK domestic law and these are further modified by the treaty. There are numerous other tax breaks for US citizens living abroad, which usually would mean that you will end up paying no tax in the USA. However, in your case, the complications may result in US tax nonetheless.
You do not mention where you were divorced (in a UK court or a US one). The location of your divorce decree will determine what legal language was included and whether the pension will be deemed alimony, which will affect how your share of the pension is taxed. Other issues include whether you take a lump sum or periodic payments. Under the treaty, lump sums from the USA are taxed in the USA, even when received by a UK resident.
You need to speak with a US tax expert who can handle treaty issues and legal issues of this nature. That person will need to read your divorce decree, and can then tell you how it will be taxed if (a) you do nothing and (b) if you make a treaty claim. Once you've had the returns (US and UK) done professionally for one year, you more than likely can copy the work for all future years.
Don't feel you have to give up your US citizenship; get help first.
