looks like a bit of a bodge job
you need to find out what cable stuff is wired in before you even consider uprating breakers
the easiest way for a diyer to size cable is to buy a small length of each size and just compare them to your existing cable i will go through the cuircuits in order you will at the very least need a multimeter (they can be had for about £10) to check that cuircuirts are rings (use the resitance range and check for a ver low value to telly you when things are connected)
ideally you should use breakers of the same brand as your board but failing that take the old breaker with you to compare and get one that matches as well as you can in the position and type of the input terminal on the bottom
6A - garage light + outside light
fine if it's an attached garage but see the comments on the garage sockets secontion if it's detached
10A - garage socket
is this an attached garage or a detached garage
if the garage is attached the cable should be 2.5 T&E or bigger if it is then you can change the breaker to a 16A one if not then you need to replace it first
if it is a detached garage it need to change to armoured cable before going underground and change back to T&E in the garage generally in this case you use a small metal cu in the garage to split out sockets and lighting from a single armoured cable if your garage is detached post it and i can give you more info on how it should be done
16A - downstairs sockets
seems unusually low but you should only uprate it to 32A if either the cuircuit is a ring (2 lives in breaker which when removed from the breaker are still linked to each other (use multimeter to check) and wired in 2.5mm or greater or if it is not a ring the cuircuit is wired in 4mm or greater if the cuircuit is 2.5mm and not a ring you can uprate to 20A but no higher
32A - kitchen (new extension) sockets
this sounds right but i would still check it as for the downstairs sockets
20A - heating + freezer socket
the cable should be at least 2.5mm and the freezer socket if in a position where it is likely to be used outdoors shuold be clearly labled as non rcd and not for powering outdoor equipment
6A - upstairs lights/downstairs lights/burglar alarm
the burgular alarm should not be on a lighting cuircuit since you are short on space in the cu i would put it on the freezer cuircuit but remember to put in a fused connection unit before it if there isn't one already as for the lights this is not against regs but is rather prone to nusence tripping you can uprate to a 10A but if you do so you must replace all swsitches etc with ones that have this rating
16A - immersion
the cable should be at least 2.5mm
32A - upstairs sockets
this sounds right but i would still check it as for the downstairs sockets
40A - shower
would need more info on the size of shower to confirm this rating also the shower should be on the rcd but this raises the issue of the current rating of the rcd if the rcd is rated at 80A or more i would move the shower breaker over to the rcd side and use the spare this would leave on the non rcd side for your loft power which is a perfectly fine location if the rcd is of a lower current rating i would use a seperate rcd with a current rating of 60A or so (so you have some leeway if the shower is uprated in future) in a small enclosure for the shower something like
this and
this
before opening the cu you should trun off the main switch
inside a split load cu like that you will find a earth bar and 2 neutral bars one for cuircuits on the rcd and one for cuircuits not on the rcd the mcbs will be attached to busbars feeding off the main switch and rcd the terminals bringing the wires in to the main swich should be seperately covered but they are not always so be extremely carefull the first time you open the cu and if they are not covered make a termporary cover and try to get a proper one asap
when connecting a cuircuit you simply conenect the live wire to the mcb the nuetral wire to the appropriate neutral bar and the earth wire (remember to use earth sleeving) to the earth bar if the cuircuit is a ring you will have two cables to connect to each terminal but again you just connect them as above
for the sockets in the loft i would reccomend using a 20A breaker and running in 2.5mm it is perfectly safe to use a 16A though if you have one spare though provided you don't plan to draw huge amounts of power up there