URGENT help required please ! re booster seat for use on plane

[TheWheelsOnTheBusHaveFallenOff]

sodding BA have only just told me I need to bring a booster seat for ds, 21 mo, who has his own plane seat booked. my car seat is way too bulky to lug round the airport so I am looking for a highback booster seat which:

is lightweight
has 5 point harness
can be secured by lap belt alone
is relatively cheap!

anyone got any ideas please - going next week and cannot find anything on the web that is definitely all of these things, so many sites do not give weight of seat or dimensions and I don't have time to order and send back and reorder!

Phone a shop and ask them, maybe Kiddicare, someone like that?

I thought car seats only have a 3 point harness? Maybe phone Mothercare or Halfords?

why do you need a booster?
can't he sit on your seat for take off and landing?

That's what I thought EldonAve - he has to sit with me for take off and landing, but apparently to have his own seat as an under-2 I also have to bring a car seat with harness etc. which I only found out today. and BA's website say they have such seats available but when you ring up the computer says no apparently. Grrrr.

I would just turn up without the seat
We used one of the airline boosters on virgin once but didn't bother on our latest trip - DD just sat on my lap for takeoff and landing

aggh such a dilemma and many thanks for your replies.

am going to order the Britax evolva 123 I think, given it is 11 hour flight and overnight one way I guess best to go with the seat and yes it will come in handy in later years!

Just taken by surprise as I hadn't factored in having to take a seat with me too as well as all our other unneccessary overpacking bags - just me and ds travelling and the airport is the most stressful place with small child, luggage and only the one pair of hands!

Loopymumsy many thanks for clarifying the law - the BA website has lots of info but is not entirely clear.