Capital gain tax in US?

If someone living in the UK is to sell a property back in the US, does that mean one has to file a non-resident capital gain tax in the US for both state and federal? Would that be sufficient  so there won't be double taxation in the UK?

How difficult is it to file these taxes themselves or is it easier to hire a CPA to do it? Does anyone know if there's such service available in the UK for filing US state and federal taxes?

From your earlier posts you say you are a USC so you should have been filing taxes every year in the US since USCs are always considered resident for US taxes regardless of where they live.  If you are using tax software to file your US taxes (TurboTax, TaxCut etc.) then the software takes you through the steps of selling a house.  Since it is a house in the USA there are no foreign currency issues to worry about.  As a UK resident you will also have to calculate the capital gain on the sale and possibly pay capital gains tax.

I take it this is not your primary home?

Assuming the property is owned by a US person, the sale is subject to US Federal & State tax. The calculations would include recapturing depreciation allowable but you would be able to set off any suspended passive activity losses. The gain would also be reported to HMRC (using spot exchange rates for purchase and sale). The UK gives a credit for US Federal & State tax due on any doubly taxed gain.