Do you own the flat and part of the Freehold or is it Leasehold, like most are? Firstly if it is Leasehold ( most likely ), you cannot alter the layout without the permission of the Freeholder. Once you have sought permission from the Freeholder, which you will almost certainly have to pay for, then you can proceed onto Building Regs. All you need to do is get in touch with your local council and they will assist you further. It will all be governed by the plumbing and what's possible!

Front hallway- I say save your money leave it up. Cost of removal is minor but you will likely have to move radiators and electrics and make good plasterwork and coving- I say protect another £1500 profit. Present the larger back reception as a living room and present the front reception as the dining room. Given that the bathroom is likely to go upstairs make the downstairs bathroom into a room to access the garden- I would present the kitchen as a kitchen/breakfast room with doors to see the garden as being able to see from the front door to the back garden should pay dividend.

The bath of my dream is spacious and preferably in the style of minimalism. I like warm colors and a lot of free space to be left. I'll use your link, thank you!

If I was to try and include one upstairs I would place it in the bedroom rather than a split (it’s also the best choice to the plumbing element):- Although this might make the bedroom too small (drawings not to scale for fitting in a bathroom suite). I‘d possibly look at what you can do on the ground floor, can you replan the space and lose some from the porch / Kitchen? Although for me, I’d fully reconfigure the extension end, do away with the porch next to the kitchen and open it up into one large usable space (as someone with a conservatory thats 3m x 3m it’s the largest most under utilised space in the property and is used primarily as a drying room in winter for laundry.... I’d look to place the kitchen in the top half and the area marked for kitchen on your plan would be turned into a utility/laundry room and toilet/shower area (again, the split isnt to scale, but you get the idea of where to place things). You can have a nice large window, folding doors or even a sky light over the kitchen to give it loads of light (can the porch be brought out the extra bit to make it level and a true ‘L’?). You could also reposition the dining room doors if you wanted to use the wall along that side for more kitchen storage. I’m no good with room design but I feel this will give you the best options for the property.