The article you have linked in your first paragraph does provide some more context to the British government's characterisation of the planned demonstrations as "disrespectful". The main concern that members of the government express is that such demonstrations might interfere with ceremonies being carried out in Whitehall, although it's worth noting that organisers claim they do not intend to demonstrate in that area. The deputy prime minister expressed similar concerns on November 5th saying that,
I think the police need to think very carefully about the safety of that demonstration, namely whether it could spill over into violent protest and the signal it sends, particularly to the Jewish community.
As a comment by Jonita points out, the Home Secretary has gone further and described similar demonstrations as "hate marches" (although the Metropolitan Police Commissioner seems to disagree). Obviously, if you believe that description, then it is disrespectful to hold such a demonstration on a Day of Remembrance (or any day, I suppose).
However, there is a broader point here which I think it's worth drawing attention to. You describe Remembrance Day as a "call for peace and remembrance of the horrors of war", which is certainly true for many people, but the British government seems to have a more limited understanding of the event. This is from the official guidance for the Remembrance service that will take place on November 12th,
Starting at 11am, the service will commemorate the contribution of British and Commonwealth military and civilian servicemen and women involved in the two world wars and later conflicts.
Similarly, the Royal British Legion, which if the main non-governmental organisation associated with Remembrance events, has the following to say,
Remembrance honours those who serve to defend our democratic freedoms and way of life.
We unite to remember the service and sacrifice of the Armed Forces community from Britain and the Commonwealth.
Somewhat interestingly, the British Royal Family, apparently has a broader interpretation,
On the second Sunday of each November, The King leads the nation in remembering those who have died in world wars and other conflicts
... two minutes of complete silence is held in remembrance of all lost in war.
My vague recollection is that, in the not so distant past, it was widely accepted in the UK that Remembrance events commemorated all victims of war, but that at some point (I don't know when) much more emphasis was placed on British and Commonwealth military personnel. If you look into the history of the White Poppy, you will find that there has been a certain amount of tension over this point for quite some time. In any case, once you realise that the government views Remembrance as referring specifically to British and Commonwealth military casualties, it is easier to see why they might regard demonstrations in relation to the Gaza Conflict as "disrespectful".
