Why on earth hijack another airliner?
It's so old hat....
And it already has done all the economic and other damage you could wish for.
Why not sail up the Thames, put a couple dozen gunmen ashore, and attack Canary Wharf, the Tower, the City, Harrods and the Tate (while you're at it).
If you use an old barge, you can also set up a whole lot of pre-targeted mortars under a tarp. And then finally blow up the barge in front of the Houses of Parliament.
CJ