complete a US tax return, where-ever you live in the world, stating your worldwide income....effectively inhibited from investing in UK mutual funds and could find it rather difficult to open a UK bank account.
If you have kept a UK bank account, it won't be an issue. If not and you return to the UK with US Citizenship, you will be resident in UK and opening a bank account will not be too difficult. Taxes can be a pain though; filing is not a problem - either do it yourself with TurboTax or similar or use a US based accountant, however, depending on where outside of the US you reside and how much your income, you may be subjected to taxation, despite residing outside of the US. Working at somewhere such as EK for example could be a bit painful but in the UK, with the currently weak GBP, it's not likely to affect you in the short term.