You can boost before your flight but although the money is taken straight away you don't actually get the miles until after your flight. If there is a bonus on, say 20% extra, you get the extra 20% immediately but not the rest.
Your best bet is probably getting a referral for either a platinum or gold Amex. The Plat comes with 35,000 miles if referred, but costs £450, you can cancel for a pro rata refund once your points hit. The gold card comes with 22,000 miles if referred but is free for a year. (There is a minimum spend on both cards for the points).
Tesco have 4000 bonus points on pet insurance at the moment (12,000 miles) the t&c's' say you have to keep the policy for 30 days to get the points but with policies at about £5 a month for an indoor, neutered cat that's a small price to pay for miles.
If you're not already a member Topcashback can get you 12,500 miles through Clubcard conversion. Buying car/home insurance or switching energy supplier are great Topcashback earners.
Keep an eye out for other Tesco promotions, try shops away, Virgin money isa, e-rewards, Virgin wine, the hotel booking sites Rocketmiles or Points hound, if you have any upcoming stays.