This probably covers most of the issues., although the return from premium bonds is not guaranteed and will probably work out less than a fixed rate or even instant access after tax, unless you're lucky enough to win a bigger prize. You could well decide it's worth the risk though (I think I would)
If you're looking to buy again within the next year or so, realistically you only have this and next tax year's tax and ISA allowances and you aren't going to invest, it's never going to be worth paying for advice because, apart from using your ISA allowances, considering premium bonds, splitting the money between you and being mindful of when interest is paid vs tax years, there's not really any more you can do. You're almost certainly going to pay quite a bit of tax, but that's because you're going to earn quite a bit of interest (£500k for a year at 5-6% is £25-30k)
Moneysaving Expert gives a full guide:
Best savings accounts: 5.2% easy access or 6.2% fixed rates (moneysavingexpert.com)
Assuming you are married and the money belongs equally to you, I'd get two lots of £50k PBs, £20k in cash ISAs to use this and next year's allowances - for this year, if you're sure you won't move in well under a year, perhaps go for a 1 year fix (but look out for the MSE advice on the little bit of flexibility re access to ISAs).
For the rest, for simplicity and flexibility, you could each open a Santander savings account and opt for monthly interest (be quick, it's only available for a few weeks or could sell out earlier) and you'll get just over 5% instant access.
Finally, last consideration is protection against banks going bust, but you don't have to worry about this in the short term as you have extra rights for six months following sale of property. After this, don't have more than £85k per person per bank (sometimes more than one bank is covered by one banking licence and here the licence defines the 'bank') but this limit doesn't apply to NS&I.