I have done UK twice on my own once with five month old and once with 13 month old and 3 year old.

Biggest tip don't stress think of it as solid time you can spend just keeping them happy. They pick up on tension so much if you are relaxed they will be.
Feed or have something to suck on for baby and toddler on take off and landing (helps avoid ear trouble).
Don't take too much stuff on plane - less is more you don't have much room and is amazing how interesting they can find the safety card and other things on plane if you engage with them about it.
Practice going on plane at home explain how it works toddler will be fascinated. Take a pillow the tiny ones on planes are not much cop.
Consider if you really want a bassinet if one is an option, personally I think over 6 months are more hassle than worth - you have to sit in bulkhead seats and these the arm rests don't go up so a toddler can't lay across your lap. You have to get baby out of bassinet any time turbulence as well as take off and landing and one trip that was practically whole trip. Bulkhead seats trays come from side and eating with baby on lap in them virtually impossible at least in normal seats the tray comes down in front so is a bit easier.