Mrs EB flew BOM-LHR yesterday with our 15 month-old, paying the child fare but with no car seat...AFAIK she didn't have any trouble from not having a car seat - obviously EB Jnr had to sit (read: wriggle

) on her mother's lap during takeoff and landing and any turbulence but other than that (i.e. when the seatbelt signs were off) I think she was fine with her own seat...
So if the worst comes to the worst and they don't let you use it, said toddler should be OK without it...
Never seen anything about the 26" thing before...ours is a Britax one and all the site said that was it was CAA-approved.
HTH
EB