We spent 3 months in the UK last year and absolutely loved it. I'm so excited to hear you all are heading there are you are sure to fall in love with the history, people, and the experiences you will have. I hope your trip includes Edinburgh which remains one of our favorite places in the world and some of the friendliest people we've met.
As for travel planning there are so many resources available today that I struggle to consider recommending a travel agent for such a trip. Obviously this is a matter or personal preference and the time you want to spend. For me, I love the planning and enjoy researching the options as it gets me excited for the trip itself.
The airfare is likely the easiest since there are some excellent sites available now. One of my favorites is
Cheap Flights - Compare Airline Tickets with Skyscanner.com. It lets you select your destination and then you can see all the fares for an entire month to see which is the cheapest. I suspect you'll be flying in/out of Heathrow, but may want to look at Edinburgh as well just to see if you can save some money that way.
As for hotels, there are more resources than we can imagine but I suggest using TripAdvisor as you can see the reviews and then launch into booking sites. There are dozens of alternatives, but this is the closest to a travel agent you can find. Though it does require researching each location.
I believe if you were to provide an idea of the length of your trip, what you want to see/do, types of activities you enjoy, and the primary goal (history, food, culture, etc) then tall of us here would love to help. We may not be travel agents but I know we're passionate about travel and love to help.