With the Nomad, at least it will fold small enough to put in the overhead compartment if they won't let you use it. The XL will absolutely not work though as it's an isofix seat, not seatbelt fitted in the harness mode.
There aren't any car seats on the EU market any more which have the TUV (European airline approval) for this age group or forward facing. You could get an older, second hand Britax Eclipse but then that's not so good at the other end as you won't know the history of the seat or how old it is.
This is what TUI's website says:
Approved child car seats can be used for toddlers or children in accordance with the age, weight and height ranges that are recommended with the manufacturer. It's the parents' responsibility to make sure the seat is suitable for their child. You'll also need to make sure you bring a car seat that...
- Conforms to European safety standards
- Is not wider than 40cm or 16"
- They usually have the reference ECE R44-03
- Is in good working order, and must be fitted with a single release-type harness
- Is secured in a forward facing position only
- Is secured by the aircraft seat belt, through rigid lower bars and not over the child
The 40cm width will be tricky, although the Nomad might fit into it.
ECE R44-03 is a really old, obsolete standard - they haven't updated their guidance in about 20 years. The current European safety standard (and Maxi Cosi Nomad) is ECE R129 though, so should fit into this, since they only say "usually has this reference". Whether or not the airline staff will know this, I don't know. You might want to print out some information about the change from R44 to R129 to show them. This happened in 2013 and it's difficult to even buy R44-04 car seats any more, let alone R44-03.
Rigid lower bars refers to a feature on older car seats where they had metal bars built into the lower part of the frame. This is rarely seen any more, perhaps today's plastics are more tough so it's not needed. But essentially I would understand this requirement to mean that the seat must have a rigid shell which the belt is routed through underneath whereas the child sits on top, so you might have trouble with the Nomad for this reason, and also because if you think about how an airline buckle is, it would be right in the way digging into the child's back and be really uncomfortable. I would also take the "spirit" of this rule to mean that booster seats and impact shield seats are not allowed.
You might be better off looking at something like Graco Extend R129 or the older Cozy n Safe Fitzroy (R44) as these do have a solid shell with the benefit that at your destination, you could use them rear facing for your child which is safer (The Cozy n Safe only if he is under 13kg).
Bear in mind none of these are designed or tested to be used with just a lap belt, so it would be at your discretion whether you feel it's safer than just a seatbelt, but their guidance doesn't explicitly forbid this usage.