Ok, not all S&S ISA's are created equal...
Interactive Investors costs about £9.99 per month and then you have transaction fees on top of that. Much better than 1%+ than you can get charged with others accounts...
Funds... not all funds are created equal....
Watch out for transaction costs, ongoing management charges, and only occasionally now, a spread. Vanguard are generally regarded as the most competitive on the market and are available on most platforms....
In addition, you have thousands to choose from, so that can make like harder.... The Vanguard funds are decent, but not stellar, Baillie Gifford has had a fantastic year with Tech and Innovation, but in recent weeks they have rolled back 30%'ish and thus only made 70% in a single year, which is phenomenal.. Don't expect this to be the same in the coming year... Put simply tech allowed people to work from home and thus those companies that could carry on as normal or were essential benefitted..
A lot of people go for index funds because its damn hard to beat an index fund, but clearly last year Tech did... The rest of the time, most people are satisfied with a 7-20% return... I like Fundsmith Equity which has its Buffett based philosophy with a bit of resilience thrown in - but this is regarded as higher risk than Vanguard.
JustETF and Trustnet are good websites for reviewing what funds are available and the level of risk you might associate with them.
In terms of £80k - ISA is definitely the place to put it if you want growth, because of the tax free growth and as its a pension lump sum, you are on the decumulation phase of that pension. However, if that pension is a SIPP, be very aware of taking on the same risks with the ISA, that you have in the SIPP, if its defined benefit, there is no overlap, so that's good.
As others have stated, most investments should be planned for 5 years minimum, although many are starting to say 5-10.... That being said, nothing to stop you cashing out after 6 months, if you are in profit and you are looking to buy a Tesla, Yacht, etc... So you have to factor in to your thinking that you may have to sell at a loss or wait a year to break even... I had a Jupiter fund that never got back beyond its 95% of what I invested point, called it quits eventually, popped it into Baillie Gifford American and doubled it... Luck rather than judgement of course...
So which leads onto the final point, which is how much ready cash do you want available.... The 80/20 (Equities/Cash or Bonds) rule is good for some people, 60/40 is more robust for others.... Im currently at 20/80 because at Christmas I knew the tech dip was coming, cashed in and kept my profits... So im now in the same boat, trying to figure out what my 80% will head into... I choose slowly, but there is a mantra than many follow.... "Time in the market is better than timing the market" and I believe this to be correct, unless there are exceptional circumstances, and I believe this Covid period to be one such period...
I'd steer clear of Bonds right now... As I would crypto... As I would precious metals... So im kind of unorthodox, Stocks(equities) make sense to me, Bonds return has been heading in the wrong direction for a long time now, crypto could be a bubble about to pop, and precious metals is the biggest rigged game in town... Im sure others here will suggest im totally wrong, but I have 7 digits in investment terms and working on making it 8, and it works for me... So you have to find what works for you...
I can't do a recommendation for you... But at this point, if I was your goodselves, I'd keep a 20k in cash, unless you have a planned big purchase in mind, and split the rest 50/50 and pop half in a Vanguard fund that appeals, and the other half in Fundsmith Equity...
In terms of the market being high, it is in places, and tech, innovation, Tesla and some healthcare is... The less sexy equities are in a reasonable place right now, so I'd be surprised if you weren't in profit by May, but levelled out during the summer and by the end of the year, had a bit more profit... Vanguard Global has 200 different companies within it, so if any dodgy ones remain, max investment is 2% ish, and Fundsmith im pretty certain has nothing dodgy in its portfolio right now.
Given the level of funds you have, you could diversify further and pop 10k in a broader range of funds.... If you do decide to do this, I'd suggest you keep the levels nice and even, eg. make it the same amount in each fund... Too often I've popped 10% in many funds, yet it was one of the ones I put 5% that had the stellar performance... But then again, I have always gone for above average risk and we have had a great 20 years of investment returns, even though we have had 3 major catastrophes in the economies, more if you include currency issues...
Finally, one area im a little twitch about is the USD exchange rate... Its 1.4 right now.... If that drops back to 1.2, be cautious of that exposure because it will hit your returns for US Equities... We've benefitted as it went up to 1.4, and it may go higher... I won't go into the politics of this... If it does drop, you will have to wait longer for your profit.
I hope my inane ramblings assist... Happy to answer any questions you might have... Ultimately, I've found that if you view investing a little like gambling, you understand the highs and the lows a little better, the only reason why its better than gambling, is that your horse doesn't have to finish first for you to get your stake money back or some winnings...