The 18-135mm will be your general purpose "walk around" lens.

That lens tells me you are using an APS-C "crop sensor" camera, which is an important consideration. Looking at your earlier posts, I confirmed this and see you're using a Canon Rebel T7i. On it that 18-135mm will be moderately wide to fairly powerful telephoto and everything in between. I assume it is either the EF-S 18-135mm STM or USM, which are both optically good (the same, in fact). The main difference being that the USM lens is quite a bit faster focusing, but that may only be important if shooting sports or wildlife action.

If you will be traveling with other people you wish to photograph or simply enjoy photographing the "locals", you may want a portrait lens. I assume what you've got is the budget Canon EF 50mm f/1.8 STM... affordable, compact and light weight... which on APS-C is ideal for portraiture as a short telephoto with a relatively large aperture (to blur down backgrounds, when wanted). It might also come in handy for low light shooting, assuming a short tele is appropriate.

A very small, light, and very inexpensive lens that might complement the 50mm is the Canon EF-S 24mm f/2.8 STM "Pancake". That would give you a moderate wide with a reasonably large aperture for low light, group shots, "environmental" portraits, etc.

And, if it were me, I'd definitely want a wider lens for scenic shots, cityscapes, architecture exteriors and interiors. I carry a Canon EF-S 10-22mm for that purpose. But if you are interested in these types of shots too... compact and lightweight for travel, plus low cost... there is the newer Canon EF-S 10-18mm IS STM.

Those FOUR lenses... EF-S 10-18mm, EF-S 18-135mm, EF-S 24mm f/2.8 and EF 50mm f/1.8 STM... weigh less than 2.5 lb. That's a full pound LESS than either the EF 28-300mm or an EF 70-200mm f/2.8!

Accessories I ALWAYS take:
- Extra batteries, charger, extra memory cards and means of backup.
- Matched lens hood for all lenses. (They reverse on the lens for easy storage. Be careful, they are very lens-specific. For example, Canon 18-135mm STM and USM use two different hoods).
- Accessory flash. The one built into the camera really sucks. There are small, compact flashes like Canon 270EX that are much better.
- A high quality circular polarizing filter. In this case, 67mm size would fit both the 18-135mm and 10-18mm... the lenses where it would most likely be wanted.
- I carry quality UV filters to "protect" my lenses, but only install them when actually needed. However, the lenses here in some cases cost about the same as the filter, so why bother?
- Macro extension tubes, which can be used with any lens to make it closer focusing, weight little and don't take up much space. I use Kenko and Canon. There are others that cost less which are fine, so long as they have the electronic contacts. Avoid the extremely cheap without electronic contacts, difficult to use with modern lenses like yours.
- Minimal lens and sensor cleaning supplies: swabs, lens/sensor pens, micro fiber cloths.
- Plastic bag(s) and cheap plastic poncho (in case of rain).
- Small, unobtrusive bag to hold it all. I prefer something that doesn't look like a camera bag. Must meet airline carry-on rules. Never check camera gear. Always carry on.

Leave the 70-200 and 28-300 at home.

Geez... there are a billion things to photograph in London and Rome... so take plenty of batteries and memory cards! And maybe get some sort of cloud service where you can upload your images, both as security for your images as well as to be able to clear memory cards for re-use. But SD memory cards are cheap... so might want to just take a handful of them and leave images on them, as well as back them up to "the cloud". Personally I choose to use moderately small cards (around 32GB) and more of them, rather than one or two gigantic cards. That way if any single card is lost or fails, I haven't lost everything.