Not sure about the English Nature list, but the reason that red-eared terrapins are not listed in Schedule 9 of the Wildlife & Countryside Act (see
http://www.defra.gov.uk/environment/gm/nonnav/06.htm ), is that they're not 'established', i.e. they're not known to have bred in the wild.
The Schedule 9 list also includes Triturus carnifex (which persist in Surrey, & have interbred with T. cristatus over a small area), Emys orbicularis (apparently still sighted in the Norfolk broads from time to time), and Elaphe longissima (still breed around the zoo in Colwyn Bay, and possibly in London). I think the schedule was last updated before the discovery of breeding colonies of American bullfrogs in Kent/Sussex, and green lizards in Dorset.
Wycherley et al's 'Tracing Aliens' paper suggested (by call identification) that Rana perezi and R. bergeri are also present in the UK.
The Rana pipiens in Bristol is very interesting- I've heard rumours that these turned up at a southern university (possibly Bristol?) in the 1970s or 80s- I think they were quite commonly used as lab animals at that point, so it may just have been random escapes rather than an established colony.
There are still alpine newts in NE England- a few were found in an amphibian survey earlier this year. They were introduced (before it was made illegal) as an experiment to see if they'd survive that far north- apparently they will.
Anyone interested in the (now extinct) New Forest tree frog colony should read Charles Snell's recent article in British Wildlife- he suggests that they may be a forgotten native (like the pool frog).
Incidentally, the Herpetological Conservation Trust are currently running a campaign to monitor alien herps in the UK:
http://www.alienencounters.org.uk/