TUI not allowing me to amend a flight unless I pay 70% amendment charges

CardinalWolsey

Consider the airline. Whilst we've all had a bad year, they've had an exceptionally bad one. If they allow passengers to cancel or move bookings on flights which are now definitely (at the time of press) going ahead, they risk running flights with insufficient passenger numbers to cover the cost of operation. Cancelling a flight (because it would be loss making) is the last resort for them - they will have passengers with other commitments across route networks depending upon their flights running as close to schedule as possible, as well as the disruption of airframes being in the wrong place at the wrong time.
Your disinclination to travel is not an insurable event, and the airline shouldn't be considered as the insurer of last resort. Depending upon how much it would cost to cancel completely, your best option may in fact be to pay the outstanding balance, and wait to see if the provider cancels - when a full refund would be due.