Partial Drawdown of pension tax-free sum to clear interest-only mortgage

mking

I am considering using a partial drawdown of tax-free cash to clear the balance of an interest-only mortgage. I’ve been reading around on here and elsewhere but would appreciate any feedback from forum members.

• I am 55 years old, married and still working, including paying into a work DC pension with contribution match.
• I don’t intend to retire, but I recognise that circumstances can change – voluntarily or otherwise.
• My SIPP consists of my own share portfolio that I have managed over years and is pretty much at the LTA
• I have stopped making contributions into the SIPP, but still pay into the work pension as above.
• I have never accessed my pension
• I have 2 ISAs for me and my wife. 
• The interest only mortgage has no repayment penalties and is c. £100k in value. It has 5 years left if I wanted to take that term.

My thinking is that I would like to clear some of that mortgage and so would like to crystalise, say, £80k and take £20k out as tax-free cash to pay down the debt. I would leave the £60k balance invested. If this worked as I expected, then I’d think of taking out more, potentially up to the full £100k.

My understanding is that this would not trigger the MPAA, because I have only taken out tax-free cash, and so my work pension contributions would not be affected. Is this correct?

I am also not too clear on the mechanics given that I invest in individual shares. For example, if I had £60k of Apple shares and £20k cash in my uncrystallised portfolio then I believe that I could instruct my SIPP provider to move the £60k of Apple shares into crystallised form in the SIPP and the £20k cash would be paid out to me. Is this correct?

However, the value of the Apple shares will obviously move in real-time and so my instruction to move, say 440 Apple shares, into a crystallised form may be worth £60k one day, but £61k when the instruction is received by my SIPP provider. This will have implications for the 25% tax-free amount and so I am not totally clear on how to manage this. Maybe it's not really an issue...?

Finally, I realise that I could use the ISAs to clear the mortgage. They have been the source of funds to bring the interest-only mortgage down in the past. However, my feeling is I would like to draw some down from the SIPP because it is at LTA. I remain positive about the prospects for my uncrystallised portfolio. 

Views and feedback welcome on any, and all, of the above. I am trying to make sure I understand this correctly and avoid making a mistake I might regret later.