You can get 600 points via a tier point run in the USA for well under £600. You could get almost 1500 tier points with. HNL SJU HNL run in first for around £900. You have to get there of course and you need 4 BA sectors. But for around £1500 and a lot of jet lag it could be done.
Ex EU fares also provide some cheap routes to silver especially IST with 80 point each way in club. Start a trip to Asia via LHR and HKG and silver in one journey for around £2000.
Better still use Royal Jordanian to HKG and you would be at 480 points for just £1100.
£700 for silver is doable but not from London……..