You are only allowed to take out £7000 (or around that it might have changed) of ISA's each year so you wouldn't be able to put it all into ISAs straight away.
Those ISAs wouldn't have to be in a pension scheme (although I think there are benefits to that - the government puts some in) but could be. If they were in a pension scheme you couldn't get the money out again. ISAs you can cash in any time but to be sure of making money you do really have to think of it as a very long-term investment.
If you go down that route, look for a company who don't charge for a straightforward account and who allow you to keep all the money made (and take any losses- IME these even out over time). I have used Hargreaves Lansdowne for 10 years and find them excellent. Banks are totally crap for this.
As to mortgages and house, you'd have to check your terms of your current mortgage to see if it were worth your while. You would probably have an early repayment fee which you would need to include in your calculations. You would want to be careful about how long your current deal lasts: if you need to change anyway soon, you might find it just as well to wait until that time to make any early payments.
You do have to be disciplined about the cash saved. I put it straight into ISAs but Hargreaves Lansdowne can suspend it for free for a few months if there is something I want to pay for.
Frittering would also be a problem if you bought a house, though. I do think the house is a good option, and also can be used as a source of pension.So, if you got £420 for it, £100 could be for the mortgage, some for pension and some for spending.
I am not an expert, btw. I just did this two years ago and this is what I thought through.