Zip & Link Twin Beds in the UK

We are looking for recommendations and comments about Zip & Link beds for one of our Airbnb lets in the UK**.
We currently have twin singles, 90cm wide x 190cm long. Agency and guest feedback suggest that we could get more lets if we could offer the twin bedroom as another double, by zip & linking beds together.
We have ruled out the sort of joining pad that Out of Eden supply as too clumsy, and are budgeting to replace the beds which are getting a little tired anyway.
Size-wise, we are looking at 90cm (3 feet) wide singles, which would obviously link to create a 180cm wide super king.
So, first question to anybody who is using zip & link beds, are they 190cm (6 feet 3 inches) long mattresses or 200cm (6 feet 6 inches), like a super king?
And second question, any positive or negative feedback about suppliers of these beds?
Finally, any comments about headboards?
Many thanks in advance.
** We appreciate that UK sizing is different to both US and European beds.