I am in the UK also.
If you can maximise your ISA contributions ever year without fail you should definitely also maximise your sipp contributions. Sipp tax relief becomes a much better deal when you are a higher rate tax payer. There are numerous articles breaking this down online such as
http://www.whatinvestment.co.uk/inve...nt-guide.thtml
http://www.moneyobserver.com/how-to-...eginners-guide
http://www.telegraph.co.uk/finance/p...-the-most.html
With both types of saving you do run the risk of future government intervention but this is higher with pension savings.
As you point out your tax free allowance is most likely going to be biggest wherever you leave the highest risk assets (such as EM exposure) for the long term. I would not plan on ever touching them until retirement. Place them in the wrapper which you wont withdraw from (even on a property purchase).
The most important thing, which I don't know if you have considered and none of those articles mention are the new ISA products available now, and coming soon. Especially in your case for future first time buyers. These are "help to buy" ISA's and the new next year "lifetime" ISA. With the lifetime ISA if you are under the age of 40 in April next year you can make a £4000 contribution and the Government will give you £1000 for free. This continues every year until you are 50 so you can get a LOT of free money. The catch is you can only withdraw on specially designated "lifetime" events. These are currently a first time property purchase or reaching the age of 60. They are consulting on other life events to possibly add to this. If you think you might be buying a property in the medium term you could open a lifetime ISA next year and contribute to a lower risk investment whilst gaining £1000 a year until you buy. You could then continue putting into higher risk assets in your other ISA and claiming tax relief in your SIPP. You might also want to read about help to buy ISAs. I am not sure if they would be a better deal for you over lifetime ISAs as I have not studied them.