How your university treats and entertains its guests are more important than what is written in a banner but, if you insist, I suggest you write it as:-
"We extend a warm welcome to our honourable guests from Britain."
(Write "honourable" with a "u" since they are from Britain and don't write "experts" because there are always some in the group who are visit-coordinators, interpreters, etc. and not experts and may feel "unwelcome")
As for the "Teachers Only" sign, you can leave it as it is or change it to "V.I.P. Room" if you are going to re-decorate it especially for the occasion.