Can I ask approximately how much you're talking about? The problem you'll have is that most of the best paying savings accounts have maximum limits of £20-£50k so you'd end up having to open several. Other consideration is the Financial Services Compensation Scheme which only safeguards your cash up to £75,000 per institution it's held in.
I had to find a temporary home for some cash when I sold my Dad's flat and to be honest the best solution I found was the NS&I Direct Saver. Only paying about 0.8%, but the whole sum is protected as NS&I is backed by HM Treasury, so it just gave me peace of mind and I didn't have to faff about opening lots of accounts!
I presume you've had a nsoe around MoneySavingExpert for best rates etc?