Agency vs In-House
Working for prestigious GroupM agencies as well as familiar brands, I have first hand experience in what the differences are working for an agency compared to a stand alone company. This is really helpful in understanding which setting you might work best in.
Agency
When working for an agency, you gain experience on multiple brands and briefs and get to focus on specific areas of marketing, such as PPC for example. In my opinion, this is great for someone in their early career, as you’re able to work with reputable brands, and begin developing a specialist knowledge. However, agencies tend to have a very high and intense workload, so the right candidate must be determined and enthusiastic – the plus side to this is that benefits tend to be high too! It's a “work-hard play-hard” lifestyle. Some of the major Marketing/Ad Agencies in the UK are McCann, Mediacom, Havas, Open… and there are so many others!
In-House
On the other hand, working for a specific brand gives you a lot broader knowledge base, as you have to take into account every type of marketing activity instead of a specialist area. However, this can mean doing smaller everyday tasks which have a less significant impact. It is a more practical role which gives you increased responsibility, and moves at a typically slower pace as you don’t have to juggle multiple brands and projects. But again, the pay off may be smaller than it is in an agency due to smaller budgets and less well-known brands (in general).