I’m sorry and congratulations. What a dichotomy. 
The first thing I’d do is set up an emergency fund with 9 – 12 months’ expenses in it. There are some turbulent times ahead, so I think it’s better to be safe than sorry. You could do this in the various vehicles mentioned above like ISA, etc but the point is to have money stashed just in case.
Depending on your interest rates I also wouldn’t necessarily pay of the mortgage either just yet. I tend to think of money in the following pots:
Long term savings - retirement, house fund etc.
Medium term savings - large emergency fund/fuck off fund, new car purchase, roof repair, once in a lifetime vacation, etc.
Short term savings - small emergency fund/buffer, quick weekend trip, birthday party blow out, new computer, etc
Paying off a nagging bill - credit cards, small loan, etc
Fun money - guiltless purchases, new bag, again new computer depending price, etc.
At least for me, that’s how I’d mentally allocate the money.