MT4 vs broker's platform

Hi all,
I’m currently in the process of opening a spread betting account in the UK.
I have chosen a broker, opened an account, and funded it. However, I have now realised that some currency pairs, some indices, and some cryptocurrencies are not available to trade on MT4 via this broker. They are available to trade via the broker’s web/app platform.
My question is, what is the advantage of using MT4 vs your broker’s platform?
Ideally, I would like as many options available to me however, I am fairly accustomed to and familiar with MT4 having traded a demo account on it for ~6 months.
I would appreciate any feedback or advice.
Cheers,
Tom