There are several reasons why you might not see many signs specifically condemning Hamas. Eg:
When people are organising an event or a demonstration, one thing to consider is "What is this specific event's specific goal/hope?" - it's then common to encourage people to focus on that goal in their signs, speeches, slogans, etc. If an event's primary objective is, for example, to say that protesters do not agree with a government selling arms to a particular country or military force, then that's the thing people will be encouraged to reference in their signs etc. Staying on-topic to present a focused message is a common approach to protests, otherwise you just end up with a lot of people with individual things they're angry about, and everyone's message gets diluted.
Hamas aren't being funded, armed or supported by the UK government. The UK government has extremely limited influence on them, or direct complicity in their actions. What would be achieved by telling the UK government that people don't like what Hamas did or are doing? Hamas is a proscribed terrorist organisation, there are sanctions in place. The work being undertaken to tackle Hamas's more insidious influences and to reduce the risks of radicalisation is taking place in a different way, in settings where it's more effective - targeted interventions, inter-faith work, talks aimed at vulnerable groups, etc.
If people want to attend a march or event and carry a sign or shout a chant about Hamas's crimes, they can. If someone turned up to an event and said "this is what I want to express today", that's fine. The same principles of freedom of speech and rules about hate speech apply to everyone. If it were their first time, or if they felt apprehensive, we could try to support someone to feel safe, and consider where in the event they might feel most supported (eg if someone has children with them, they might want to walk or sit with other people with children).
If anyone feels unwelcome or anxious, they should speak with an event steward, and if anyone is threatened or attacked, they should report that to the police.