Don't bother purchasing Avios... try this instead.

Tesco are currently offering 20,000 clubcard points on certain contract mobiles. The cheapest of which is £25/month for 24 months, Total price = £600

20,000 clubcard points = 48,000 Avios points - for just £600. A far better deal than buying Avios, with the added benefit of getting a mobile you can sell, and of course, 'credit'.