hey hey afjones
as Cjean and acraven mention, more info needed.
who are we and how many, if kids ages. is that total 14 days counting arrival and departing day which leaves 12? best to count nights: 5 days, is that 4 nights or 5 nights to visit family? train london to devon and back is 2 days which is now 7 days used. how many days in london? then you want to piggyback 2 more countries. you're asking too much, unless you like spending time in train stations, trains, airports, planes, checking in hotels usual time, 2-3pm, packing up next morning, shower, breakfast, checking out, getting to transportation for next destination.
don't know what next july 2021 will bring, everyone wants to travel and hoping next summer covid-19 will allow us. summer is sooooo busy and crowded, expensive, supply and demand for hotels/apts in large cities. have you spent time in london and what would you like to see that you haven't. have no clue if all hotels/apts will be open, some may have claimed bankruptcy, may charge more to make up for loss or discounts just to be happy to have guests. what ever you decide check cancellation policies for refundable options and have insurance, read all the fine print.
are you flying multi-city (into one city out of another) to save on back tracking.
look at eurostar.com to paris or brussels, spend couple days and fly home from there. book early for better rates, fly to dublin with flight to USA from there. figure out your costs and if that's what you want.
staying one place in london, explore the city, take couple day trips (brighton, windsor, boat ride up regents canal or the thames, street markets, theatre shows, london walks)
what does nice hotels mean, rooms are smaller and stars* means paying more for amenities.
don't want to be delores downer, lots to think about and consider, what's your budget in pounds, interests. is your family advising you about your holiday?
feel free to come back and ask more questions, get answers which may be good bad and ugly to you. it will help you out knowing what your plan A to plan B and not stress out. enjoy