I have just looked up the British Waterways Act 1995, which I believe is still the relevant legislation.
The only mention of displaying anything is on page 24 where it states:
Display of certificate
10 The certificate shall be displayed on the houseboat so as to be clearly visible from
the outside of the houseboat at all times.
It is interesting to note that
on page 11, “houseboat certificate” means a houseboat certificate issued under the Act
of 1971;
and separately
and on the same page, “licence” means a licence issued by the Board in respect of any vessel
allowing the use of the vessel on any inland waterways;
indicating to me that it is a requirement to display a certificate but not a requirement to display a license.
The Act clearly treats a certificate as different to a license.