What to do with 100K just sitting there?

Heedtheadvice

Yes, nothing is risk free but owning your own home as a long term time scale consideration should be a priority.
Even if you do not wish to live in London long term you can live in it short term, save on rent (use rent money to pay for mortgage?) and then have a saleable asset when you wish to live elsewhere, no capital gains tax to pay(current system/main residence) some security of tenure......etc. etc............but some costs too. However overall usually a winner andvwhen you do come to sell in 15 or 20 years time property elsewhere in the UK is usually cheaper to no rent when a pensioner and a stash of cash too!

.......well typically anyway!