You might travel well without phone service, but having it can greatly expand your experiences.
Are you traveling from the United States or another non-EU country and into the UK? Are planning to stay in the UK or perhaps in the UK plus EU for a few days? If so — yes, even for just a few days — it can easily pay to get yourself a local SIM card and local mobile service. It certainly does if you travel long-term.
But what service and what plan? A contract (aka post-pay) is out. Should you get pre-pay or pay-as-you-go? What happens if you run out of calling minutes, texts, or data?
This is the narration of my own experience getting cellular service (aka mobile service) as I travel in the UK.