They are allowed, it’s just that people don’t like it, or actively disagree with it.
I would imagine that, even beyond recent noticeable lurch to a jingoist/ fascist tendency, there is also residual distaste towards (or general loathing of) things connected to the British Army/ British establishment.
Plus, you know, the whole partitioned and garrisoned part of Ireland, and the overt/ covert role of British servicemen in that.
And the Falklands/ Belegrano (?) and all that.
And Iraq, and Afghanistan.
Unfortunately it’s hard to separate the individuals from the organisation. You can have nothing but compassion when you read about the lot of the average Joe in the trenches, but it’s difficult to avoid it becoming a political opportunity, either way.