There are some great tax advisers in the States. However for most Americans in the UK who feel that they want to use a tax adviser, it is almost always vital to use a dually US/UK qualified tax adviser to ensure that the optimum tax position is reached.
In a practical sense it is typically helpful to use a UK based adviser because:
1. He or she will be in the same time zone and so you can meet face to face.
2. The adviser will be familiar with UK tax jargon and all the changes that the UK government constantly make to UK tax rules, UK tax credit rules and issues around UK pensions and savings (and weird concepts such as notional tax credits and taxation of US LLCs and UK LLPs).
3. If something goes wrong there will be a UK based professional body that will have jurisdiction to listen to complaints such as The Taxation Disciplinary Board
http://www.tax-board.org.uk/ - or all else fails a court in the United Kingdom where a claim could be made.
4. It is a criminal offense in the UK to provide tax advice if the adviser is not supervised for anti-money laundering purposes. To be 100% sure that any tax adviser is as clean as a whistle one should, therefore, always ask any tax adviser who they are supervised by for anti-money laundering purposes.
I concur that the prices charged do indeed vary dramatically here in the UK from £100 or £200 up to several thousand pounds, so it is fundamental to check what one is buying by checking out professional qualifications.