Wikipedia suggests there's no imbalance.
There has been criticism of the Extradition Act 2003 in the UK in respect of the cases of Gary McKinnon and the NatWest Three.[19] In particular, the Act has been criticised for reducing the level of evidence required for extradition from the UK to US from prima facie evidence to 'reasonable suspicion', and for allowing extradition to proceed on the basis of offences in US, rather than UK law. In addition, the standard of proof required for extradition from the US to UK is different, in accordance with the Fourth Amendment – the standard of 'probable cause'.
When in opposition the Conservatives and Liberal Democrats criticised the Extradition Act 2003 and in September 2010 Home Secretary Theresa May an independent review of all extradition arrangements was begun.[16] The review, completed in September 2011, concluded that the 'reasonable suspicion' and 'probable cause' tests had 'no significant difference', and that there was no imbalance in this respect. Moreover, the UK extradition procedure was found to be more elaborate, and more difficult to achieve, than that from the US. In respect of the NatWest Three, the report noted that the extradition evidence had been prepared according to the standards of the pre-2003 Treaty, and that was therefore no grounds to criticise the 2003 Treaty in respect of this case.[24]
http://en.wikipedia.org/wiki/Richard_O'Dwyer