I think it will be "manageable" for you to pull-roll step by step, thus in front and to the side holding handle bars and walking up stairs, one step at a time with the bicycle, thus NOT PHYSICALLY LIFTING IT OFF OF THE GROUND, only pulling on and rolling it up ONE STEP AT A TIME. EVEN IF YOU HAD A 50 POUND BICYCLE, WHICH YOU WON'T HAVE, you will be able to do that. Any bicycle that you choose even if it a 70 year old Raleigh with Headlight, generator, steel basket in front and steel carry rack in back, full steel fenders, steel wheels, steel old timey pedals, heavy spring steel seat WILL ONLY WEIGH AT 45 POUNDS AT THE MOST. You'd likely be looking at somewhere between about 37 pounds and 44 pounds for any old basic tourist style bicycle. Since you're in the U.K. , you can convert the weight to kilograms to give you a better idea, but the weight is manageable. Paul McCartney had a famous song called " Let Me Roll It ". Thats just what you gotta do.
Something with 590mm wheels or 559mm wheels might be a tiny bit easier to Roll Step By Step than 700C (622mm) wheels or 27 inch(630mm) wheels.

My advice is to buy something that is durable, dependable, and comfortable to ride, while at the same time NOT SPENDING MUCH. Why? Because you can find plenty of tourist style, rugged, almost unbreakable, older but heavy by road warrior lycra-spandex jockey standards. Certainly have a nice comfortable new seat and new tires and tubes, etc, installed if necessary. You should also consider an old style full size rear view mirror mounted on your handlebars. .. perhaps a wicker basket with a folding wicker top and metal latch, like an old timey purse, or picnic basket.....so that the front wicker basket closes and latches closed... A nice seat saddle bag attached to the seat and a rear rack over the rear fender should be useful.......you might wish to affix a sturdy plastic "milk crate" to the rack with wing nuts and fender washers, etc. I don't know if anything like that would actually be practically useful but it might give you more ideas that might be.
You might consider a step-through frame, or mixte frame. They are great and will be widely available and do not bring the resale prices of the same model's diamond frame. You don't have a car so that the difficulty of mounting a step-through or mixte on a car's carry rack will not be an issue for you.
I would say get something durable, stable, comfortable, dependable and inexpensive. SAVE THE REMAINDER OF YOUR CASH! In the future, you will know more about what you might think you might prefer. The little money that you spend on a dependable old ride will be well spent. It will leave you with CASH to have more fun with your friends, or whatever else. You will no more later on in the future if you wish to buy a new bike at that time. My bet is that you will probably be happy with whatever nice old bike that you will find soon. You seem to know what might work best for you and my only advice is for you to trust your own instincts on what you'd like to acquire. Do be careful when you respond to BIKE FOR SALE ads that are placed by individuals. Because you can never know for sure what kind of person placed the ad for bike for sale. You cannot tell based on the person's appearance or the neighborhood in which they reside.........ALWAYS RESPOND CAREFULLY WITH SOMEONE ELSE GOING ALONG WITH YOU. You want to do the purchase transaction in a SAFE public area with cameras(video surveilance) and policing. Be very cognizant that you do not provide, too much information, private information because you just never know, and you cannot take that chance, even though 99.8% of people are good. That also is my only concern, is that rolling your bicycle UP 24 steps (two flights of 12 stairs each) likely will take you as much as. about 2 minutes, guessing at about 5 seconds per step. This might be a concern for me, for my wife, daughter, or grand daughter doing this in a residential apartment building, campus dorm, beachfront condominiums, where there is not adequate lighting and security there. What I mean is for example anything much more than a minute entering and exiting opens someone alone to an increased probability to being a victim of a crime if there is not adequate grounds security and video surveillance.
Taking too long to ENTER and EXIT could be problematic in a building, especially in any downtown area or a large apartment or condominium complex without gated access, where non resident visitors must register or be announced by the gatehouse guard.
You'll know what will work for you. I don't wish to discourage you in any way. Just like you need to wear a bicycle helmet, just in case that you do actually need it. Same thing with the security & safety concerns, you should always be aware of your surroundings & your social media information disclosures and remain safe.
I know that is a downer on an otherwise positive upbeat happy posting on getting a bicycle that you'll enjoy, but it unfortunately is a necessary consideration in this day and age that we live in today.