A good freelancer suits a single-location business with a modest budget and a clear set of jobs. An agency suits you better if you need content, technical work, links and reporting together, or want cover when one person is away. Judge either on scope, ownership and reporting, not on size.
Freelancers often cost less and you deal with the person doing the work. The risk is capacity: holidays, illness or a full diary can stall your campaign.
Agencies spread work across specialists, which helps with technical SEO and content, but check who actually runs your account and how many others they handle.
Either way, keep your Google Business Profile, website, Search Console and Analytics in your own name.
Useful pages
This is one part of a bigger picture. Local SEO services in the UK: the full guide covers what local SEO includes, costs, timelines, agencies and red flags in one place.