I agree with the PP who said be careful what tour group you travel with, and also about Rick Steves. I was booked to travel with a large group around England, because I was afraid to travel there by myself. I arrived the night before our group met and wandered around London a bit. That alone got me over my jitters of traveling alone.
When I went to Heathrow Airport to meet up with the group the next day, I realized I
really didn't want to be stuck with most of the members of the group

for a
full 11 days. 
I went into the bathroom to pull myself together, and somehow, when I came out, the group had left without me - only my bag was left in our meeting spot.

I suppose I could have run, to try to find which exit they went to board the tour bus. But, for some reason I hesitated.
I ended up blowing off that trip, (and the non-refundable money spent,

) and used what little money I had left to book smaller 1-3 day tours here & there, planning my own itinerary using Rick Steves tour books & the tours in the British Airways travel guide, in my own time frame, staying in lovely B&Bs, and completely had a blast! I even went to Paris, thru a Chunnel tour for one day. (Much too short a trip.

) But it was totally unexpected, and I loved it.
I now look at that original tour I booked as just the security & means of
getting me to England, and over my jitters of doing it without a solid plan. (I was such a scaredy cat.

) But, once I was there, I realise it wasn't like traveling to Mars. They're
humans. 
I even traveled on both London's & Paris's underground subway systems on my own, without worrying I'd end up God knows where and no clue how to get back.
I now want to go to Ireland or Scotland. I'm pretty much going to do the same thing. Smaller, individual 1-4 day bus tours or self-drive tours, visiting only the areas I want. And not getting stuck for too long with any one group or tour. Spending longer, where I want. And I may ferry to Wales

& head on towards London.
England and Ireland are really smallish islands, compared to traveling in America. To travel by car between London and Dublin, the trip from London to Holyhead in Wales will only take between 6 and 8 hours, depending on the traffic. That's if you make it in one sitting without any stops, and who wants to do that? Then, it is then a two-hour ferry crossing from Holyhead to Dun Laoghaire, Ireland. Or, traveling from the southern end of England to Scotland is only a few hours longer. I personally, can't imagine taking a cruise and missing the land sites through England, Scotland or Ireland.
Some U.S. states are bigger than whole European countries. I had a boss who was Swiss-French. He spoke fluent French and Spanish and some Italian. When I asked about this, he said Switzerland is so small that the joke there is: When you lay down, your head is in Switzerland, your body in France & your feet are in Belgium. It's very routine for Europeans to speak many languages because the countries are so small. Makes for easy traveling, especially with the Eurailpass others have mentioned.
And I agree about not getting on some super commercialized tours. They get kick-backs from certain stores & hotels who recommend them. My friend is a concierge, here in NYC, & I would never dine at the places she recommends. They are overpriced, touristy & lack our real, local, hometown atmosphere, foods & prices. They aren't bad places, but when you live here, you know better. She recommends them because of the tremendous kickbacks in free services & meals she & the other concierges receive.