Thank you Jay and Waldemar (I'll keep your info for a possible future trip to the 24 Hours of Nurburgring, one that is firmly placed on my "to do" list

).
So, From the UK to Portugal, through Ireland, France and Spain, we got:
UK - Helmet NO / Seatbelt YES
IRELAND - Helmet ... / Seatbelt ...
FRANCE - Helmet NO / Seatbelt YES (reflective vest)
SPAIN - Helmet ... / Seatbelt ...
PORTUGAL - Helmet NO / Seatbelt YES
About the reflective vest in France, are you sure it applies to a tricycle? I just checked the portuguese code and it specifically mentions that two and three wheeled vehicles are exempt of the need to have reflective vests and triangles in them.