Three return flights between GLA and London would bag you 60 TPs. Flying to Gatwick would be cheaper than Heathrow. Flying on to Jersey, for a day trip, might not be much more expensive (and would add another 20 TPs).

Have you booked your trip to Venice yet? If not, look at booking a flight + car option on the BA site. Select Business for your flights and source your hotel separately. It might prove cheaper than what you're seeing now for the flights plus hotel option. If it is cheaper then skip the car hire - just don't bother turning up for the rental.

Also, look at the BA forum. There's some good advice there, particularly:

BA Tier Point runs | 2014 master thread

There will be a new thread for 2015 any day now.

Palmer