Just to update this with my findings
For trading in an ISA there are a number of discount ex-only brokers and for me, at the time of writing (Aug 2013), iWeb looked to be the best all round compromise. £5 per deal, online execution (with phone backup), full suite of instruments and real-time pricing on UK. iWeb is a discount arm of Halifax share dealing, so they have some pedigree.
For trading in a SIPP its slightly more complex - straight equity trading can be done at great value for money through Sippdeal, iWeb also have a SIPP offering for straight equity trading and all the discount brokers with SIPP offerings are also the scheme administrator. This means that if you already have a scheme administrator you cannot appoint a discount broker with their own SIPP product as a nominated broker, you have to go direct to the discount broker.
If you want to be able to trade more exotic instruments (CFDs, Forex, Commodities etc.) it can be done but a scheme administrator is required that will allow the appointment of the broker of your choice. The choice of administrators is bewildering and there are very few resources on line to be able to compare them side by side.
Independent Financial Advice (IFA) Nottingham | Best Buy Savings Tables | ISA's | Pension Annuity Calculator rate a mention here as this is one of the few websites contianing comprehensive information on a big range of SIPP providers. After a good deal of crunching, I shortlisted three SIPP providers that appeared suitable for me and these were Corporate & professional Pensions, Curtis Banks SIPP and InvestAcc Minerva SIPP. There are plenty more that will do the job, its just a question of checking they will allow what you want and assessing how much they charge for the privilege - everyone is likely to have their own specific circumstances and needs here, so (as always) DYOR. I plumped for one of the three listed above and transferred my SIPP from my existing provider / sceme administrator (which was remarkably painless).
With regards brokers for SIPPs, again there is a wide choice depending on what you wish to achieve. Assuming there is a desire to trade Forex or currency pairs directly and CFDs, those already set up for handling SIPP accounts include Alpari, FXCM, GFT, and SaxoBank, with available margin levels varying between them. They all offer MT4 if that is your bag. It is then a question of opening an account via the SIPP provider and lodging funds. I plumped for one of the ones above. IB also rate a mention here as they too have a SIPP compliant account, offering a huge array of tradeable instruments with the attractive IB levels of commission etc. although they won't allow FX trading (in my understanding) - hence no good for me.
I will update this in due course to expand on my real world experiences of the platforms, execution and functionality, but the above may save some fellow traders a little time if looking to trade forex in a SIPP.
