Tighten screws on toilet attach to vanity unit

The vast majority of toilet seat are fixed from above, remove the seat by pushing in the release button/s located on or around the horizontal pivot bar,and you will be left with two chrome shrouds each around a vertical shaft/ post, lift up these shrouds to reveal the fixing screws and tighten, ensuring you keep checking the alignment of the seat, then reverse fit the covers and seat Hope this helps.

There is likely a button under the seat, or on the two hinges that allow the seat to be lifted off. Once out of the way lift the chrome caps covering the bolts that go through the pan (easiest when twisting at the same time) below those should be a screw that can be tightened from above the toilet. Toilet seats are never fitted pre install of the pan, they can always be tightened from above.

Usual but not always, the toilet seat on 'back to wall/vanity unit' toilets are top fixing seats. If this is the case then there will be a release buttons or catch on or near the seat hinge. Some seats pull off (up) and some slide horizontally towards the front to detach. The pull off (up) type will have a cap over the screw head. Remove these to expose the screws that hold the seat hinge. The slide type will expose the screws when you slide the seat off. If for some crazy reason the installer of the toilet installed a bottom fixing seat then you will have to remove the side fixing screws at the sides of your toilet near floor level. These usual have a plastic screw cap covering them which can be removed with a flat head screw driver. You should be then able to carefully slide (slightly lifting) the toilet forward so that you can gain access to the rear and underside of the toilet seat fixing holes. Good luck

This depends if top fix system has been used . If the old way has been done then only re moving to pan with you able to get to the nut at the bottom. If this is the case I would purchase some top fis fittings and change to this system.

Glue it down