As requested, this is about Computer Science PhDs in the UK.
First, how professional life as a PhD student looks like will depend a lot on the supervisor. As such, it is very important to have a chat with them to see whether expectations align - and to understand that expectations go both ways. That said, there are some general features that will typically be present in some way, and knowing these should help asking about the right aspects.
Most of your time will be taken up actually doing the research. That includes reading up on the literature, discussing science with your advisor and others, potentially programming something or even creating some physical artefact, academic writing, and just thinking hard.
There will probably be some training opportunities (how to give presentations, how to write a paper, etc). Asking a lecturer for permission to sit in on their lectures is also acceptable. Some of the training opportunities may be mandatory, and maybe you're even expected to complete a module or so. This will be more extensive if you are part of a CDT (Centre for Doctoral Training); in other cases you may need to take the initiative to even learn about these opportunities.
There will be seminars, both on the departmental level and for smaller research units. Here you'll be able to listen to researchers (both from your institutation and from outside) to give talks about their work. Expect to be asked to present yourself. Sometimes there will be group meetings, where everyone working in a particular area meets maybe once a week. Journal clubs can be a good opportunity to get into reading the literature. These are typically organized by PhD students, so if there is none, maybe you can start it.
If you like travelling, then conferences are a great part of doing a CS PhD. There are national events such as BCTCS, but the primary focus tends to be on international meetings. Here you'll meet the few other people in the world that are enthusiastic and knowledgable about the same arcane aspect of computer science, and get to see something of the world in the process. Getting funding to attend a conference usually requires having a publication accepted for presentation there. How many conferences you'll get to attend will vary - I'd reckon that 2-10 during a PhD are usual values.