Use deep backboxes.
35mm for sockets, 25mm for lights (35mm if a dimmer).
Use large oval. Cable pulled off a reel retains a curve albeit of much larger radius, when pushed through oval people tend to kink this curve out resulting in a zig-zag down oval. Either use large oval to accommodate this or always fit two runs of oval even to light drops (permits loop-in- switch, neutral present, rather than loop-in ceiling-rose). Capping is the work of the devil if you need to replace the cable (sods law says it can be difficult, always use oval behind tiles just re time/risk of removing/refitting a few).
Use 25mm round if multiple cables.
20mm round is good for a few cables, but nothing beats 25mm if you have the space. Use a holepunch to enlarge 20mm holes where necessary to take 25mm - particularly larger MK Grid.
Preform your cables. Sheath terminates inside the box, sweep insulated cores around the bottom & back up to the L-N-E terminal positions, then bend them outwards ready to enter the terminals. Fiddly with 1G (use 47mm if 3 cables), easy with 2G (35mm ample for most situations with 2.5mm). When entering terminals verify conductors have not stacked, particularly if there are 3 because one will pop out. Obsessives align the L-CPC-N of the cable to match the wiring accessory when dropping down the oval so no crossing over and neat. Obsessives will get everything preformed, fit, push back, pull out to check nothing pulled out, retighten, push back again.
Check the backbox is shaped right, holes ok. Some fused connection units (spur) are quite wide bodied inside and any "dented-in" box can cause the lugs to obstruct fitting. Quite a few backboxes have poor/spinning earth terminals or 1-thread/stripped lug terminals. Some like to bend unused lugs back (pliers) if several cables as it prevents insulation getting nicked.
Double insulate the cores. It can arouse suspicions but in tight 1G boxes or grid it can be worth sleeving bl/br the existing conductors with flexible but tight fitting sleeves. If you do snag with a screw the sleeve takes the hit and not the insulation.
Do not overtighten on 1.0/1.5mm FTE CPC. The 1mm CPC is easily flattened such that light flexion will cause a break (typically as you push it backwards). Likewise small conductors can miss on terminals and slip-sideways past the screw so become loose. For these small conductors loop them so there is a double- contact area which will permit sound tightening without crushing.
Plaster is a lot stronger than you think. If you ever think you may need to access a block of cables, to replace one or add one, make provision for it re trunking or flexible/rigid conduit stopping short at corners with capping to cover.
Corner protection of cables. During decoration people often run a knife down a wall corner, a fresh sharp blade can go deep into plaster so if you can't get the depth stick some broken oval or plastic L-angle into the corner. Most trades do not know the 150mm rule.
Conduit between boxes. In a room you may do double vertical drops to sockets etc, but also put horizontal conduit links in where the wall is flat. You may never need to add another wiring accessory, but it means if you do it is just a case of slicing through and inserting a backbox accordingly.
Basically preform your wires so the conductors simply fall into the wiring accessory - rather than dragging a rats nest of cables around which are much more likely to get snagged by the cover screws.
I did once try cutting plastic sheet slightly longer than a backbox height and snapping it into place so it bowed out past the lug terminals - no cable could ever be snagged. Instead I simply use low depth screws, and form cables away from the screws. Double insulating is worth doing, it does prevent lug terminals or screws damaging the insulation.