If Thompson only allows for 5kg then you may want to fly with another company. 10lbs, some womens purses weigh more then that.
I think that after this comment I may leave this thread as I appear not to be getting my point across!
All airlines, not just Thomson, are the same. In fact Thomson are quite generous by todays standard in that they allow an extra 5kg (making a total of 25kg max) if you are a diver carrying diving gear. But this has to be carried in the hold. On top of that you are only allowed one bag, of a certain size and weighing a max of 5kg (although that may be 10kg I don't remember off the top of my head) to carry on to the aircraft. Although I don't remember the exact size and weight measurements it most certainly would not amount to the size and weight of a Dive Caddy full of dive gear and clothing etc. And these limits apply irrespective of the type and size of aircraft!
Perhaps you are refering to US domestic flights where the limits may be as you describe. I don't mean to belittle or insult you but have you flown on an international flight outside of the states? I'm beginning to think that possibly you haven't because these limits I describe are common elsewhere.
Believe me if It were not the case as I describe I, and loads of UK/European divers would buy a Dive Caddy asap!!
Here is something that came up via google.
Size and type of hand baggage allowed
Hand baggage is luggage that you carry on the plane with you, while hold baggage is luggage that you check-in.
The Department for Transport has set the maximum size for an item of hand baggage as 56cm x 45cm x 25cm, (17.5"x 22"x 9.85") including wheels, handles and external pockets. However, some airlines may only allow smaller bags, so check with them first.
Airlines may also have their own rules regarding the number of bags you are allowed to take on the aircraft. It is best to contact them if you have any questions.
Air travel hand baggage rules : Directgov - Travel and transport