This is a Conservative government that are reducing the dividend tax threshold. Tory governments tends to be more pro-investor. If labour win the next general election (which they could well do with all the Brexit shenanigans!) then dividend taxes could be completely reformed. See this article about the 2018 labour conference.
ISAs are a long term play, I think if you are investing small amounts of money it’s fine to not use an isa go the first few years if it’s cheaper to do so (such as with the Freetrade GIA) but I think once you hit that first 20k of portfolio my recommendation would be to get yourself set up with an ISA. Even more so if you plan on increasing your monthly contributions.
The end goal of an ISA is very exciting in my view, if you have a large portfolio of a few hundred thousand that you have managed to slowly build up over many decades, dividends would always be tax free. That’s a pretty big deal when you think about it.