For £20/mo you'll be lucky.
Even Three, the network for heavy data users, only offer 30GB at that price point and even then only on a twelve month contract, it jumps to £25 for one month contracts and payg.
They'll give you 100GB for £27 on a twelve month contract only, or unlimited at £30 a month on a twelve month contract or £35 one monthly and payg.
Outside of Three you're looking at 40GB at ~£40/mo as the highest data allowance, with 20GB maximum being available at the price point you're talking about.
The UK mobile market is improving, but you have to be realistic in your requests/demands, and that means looking at the market and seeing what's available because giffgaff aren't the network to lead the pack on this kind of thing, for that you've gotta look to Three, because they're the ones who would do it it financially viable, but generally speaking, if you want a lot of data you've gotta hand over more money.