Grant, I've looked a little into these issues as my wife and I live in London and she is a US citizen. Definitely worth taking your pension up to the point that your employer matches; as you say there's no way to get to it before normal retirement age but it's free money and will have plenty of chance to compound. Whether you can use salary sacrifice depends entirely on if your employer offers it but if they do it helps save a little more on tax.
The bad news is on the isa, of course do your own research on this but our experience has been that they are not a good idea for several reasons:
1. The IRS doesn't recognise them as tax advantaged so your us taxes will treat it as a regular taxable account
2. Thanks to FATCA most reasonably priced providers simply won't offer accounts to US persons. The providers who will touch you mostly will charge relatively high fees
3. Any non-us mutual funds, even index funds, will likely be treated by Uncle Sam as a foreign trust or PFIC with resulting nasty compliance and tax implications and possible penalties. There is some dissent that you may be able to avoid this but it seems pretty unclear.
Overall given the choice you're probably far better not bothering with the ISA. Instead keep paying into your IRA (assuming you still have earned income if you take the foreign earned income exclusion) or into a taxable account. To be safe probably best to keep your taxable account in Usa.
The UK should respect the tax shelter of your IRA and 401k, and if you're only here for a couple of years you shouldn't need to worry too much about UK taxes on your U.S. Taxable savings. I won't go too far into details here but can give some more details of our experience if you want.
Lastly be prepared for both your UK and us taxes to get pretty complicated. Have a look at
http://www.greenbacktaxservices.com, we used them to help with my wife's taxes last year and they were great and much better value than the cpa we used previously. They have some pretty good overviews on expat tax that you can download for free.