Very valid questions, you could find some relevant discussions about Revolut and Dabbl (as Rod suggested above) specifically.
Generally, I would have thought more brokerage firms will establish sooner or later, mainly depending on an increased demand across the UK population.
Hard to say yet, there are no challenger brokers with a big customer base yet so HL and others probably do not think of them as a threat (yet).
But if we look at banks, Monzo is about to hit 1,000,000 users and on track for 3,000,000 next year. Hence you could see that the legacy banks started reacting and want to replicate those startups in some shape or form.
It may happen to the brokerage firms too, but I do not think that a startup that is created in the vacuum of fully operational systems and endless financial and human resource support will have a chance against self-made startups like Freetrade.