I've done quite a bit of thinking about this lately. In a way it's good that we have some great tax-shielding options here in the UK.
Current thinking is to prioritize tax efficiency over flexibility, so priority is:
Pension (sal sacrifice)
SIPP (I'm too old to qualify for a L-ISA)
SS-ISA
Cash
Aiming to max out the 40k limit between the Pension & SIPP, don't know if I'll be able to completely fill up the ISA allowance.
Long term plan is to run hard to at about £250-300k in retirement accounts, which will probably take me at least another 5 years, as I figure that is how much you can get out tax-free, whilst building the more flexible ISA account as and when I can. We will be taking on some major mortgage debt in the next couple of years too as we plan to move trade up to a bigger house, so some sizeable cash savings will need to figure in our plan too. Our current flat is fully owned and will be turned into a rental to offset the new mortgage.