I've picked up a Pashley Classic No.33 cargo trike and am turning it into a coffee trike. Once I've got an extra 120kg + my weight on there I'm aware I'm going to need a pretty good braking system. It currently has 2 v brakes on the front and a rear hub brake on the back. What would you recommend doing to the bike to make the braking system better? All feedback would be so appreciated as I'm a little lost!
Your bike has chromed steel rims. Regular brake pads will provide not enough friction on that surface. If you want to keep those wheels, and keep rim brakes you have to get brake pads that are specifically made for chromed steel rims.
You could also replace the rims with aluminium rims. Most brake pad suggestions are for such rims. Good brake pads like KoolStop or SwissStop salmon coloured pads are indeed much better, especially in wet weather.
Better dual pivot brakes, new cables and outers, and stiffer brackets to mount the brakes will also improve your braking somewhat.
All the good solutions here involve frame modifications.
A number of tadpole trikes use disc brakes on their front wheels, and a number of 6-bolt stub axle disc hubs exist for the purpose. A path you could investigate here is whether any exist that could take the place of your existing front hubs, possibly by way of modifying the axle system used. If so, it's possible that getting IS tabs for the calipers is as simple as someone making some up out of steel plate and welding or brazing them on to the frame, or making ring-type ones that get brazed to the axle, provided the axle would be sufficiently resistant to spinning. At that point you're just dropping in new hubs and deciding on the caliper and lever configuration you want. (Staying with the split lever has a lot of disadvantages, the major one being that dividing input force from your hand into two different brakes on a cargo application is basically bad). If you did switch to dual front disc, there is the possibility of using the rear drum brake as a drag brake controlled by a thumbshifter etc.
Edit: There are also stub-axle drum brake hubs that might be able to retrofit here by way of modifying or replacing the front axle currently present. They have a torque arm that needs to be attached to something stationary. It does appear that the stub axle they come with could have a correct diameter axle shaft like the one on your trike running through it, and then use a big nut on the outside in place of the plastic shield it comes with. They come in left and right versions.
There's a cottage industry of framebuilders of greater or lesser legitimacy who are used to dealing with big, unique bikes and other HPVs, many of which are working cargo machines such as pedicabs (which have some of the same concerns as you're facing, and are an HPV type that is usually repaired rather than replaced). You could shoot some photos to someone that does pedicab repair and see what they think. They will need the particulars of the stub axle attachment system, i.e. what the bare axle looks like and all the dimensions.
It's possible to imagine going to a nicer rear drum brake and/or upgrading the front calipers. The trouble there is the risk that even after a lot of hassle, the improvement might only be marginal as far as cargo usage is concerned. The high end of the Shimano drum (aka roller) brakes are very nice, but it would still only be a rear brake. Likewise, even the nicest and stiffest sidepull calipers that could replace yours still may not be what you need for loaded use.
It would be a significant undertaking, but you could rebuild the front wheels with drum-brake hubs. These would offer a big improvement in braking power. Drum brakes use torque arms that need to be fixed to the frame, so you'd need to make sure there was a way to do that--it might take a little improvisation. I would check with a knowledgeable bike mechanic about this.
If you did this, you would entirely remove the existing caliper brakes, which look to be low quality.
With the current setup flexing of the caliper mounting backet will change the pad angles and likely degrade braking (pressure would be on opposite ends of the pads). A length of angle iron extending across the width, bolted to both the front brackets to stiffen the brackets and may be a cheap and quick fix.
Look also at the lever activation. If you are not using full stroke of the handle, you may be able to move the rod closer to the pivot point of the handle and get more brake force. This will require more movement in the handle, so you want to be certain you still have enough, however this is a simple job requiring one drill hole and a split pin so would be worth looking at. Angle of that lever looks far from ideal, you could be missing out on a lot of braking just from not getting maximum pull on the brakes. You could look at replacing it with a pair of standard levers operating the brakes independently and if that improves braking (Use a donor bike from local recycle center for parts). If so, then it indicates it could be worth spending effort on the lever(s) rather than/as well as the calipers.
It would also be possible to add another set of calipers to the front wheels - on the trailing edge (may require removing or modifying the fender), with a second lever, or activated by the same lever with a cable balancer.
I'd very lightly scuff the braking surfaces of the rims, as they look to have a mirror shine - not good for brake pads to do their job.
And you won't believe how much difference does good brake pads make. Most people change their brakes, but it's actually the pads that do %90 of the work.
It's also the cheapest.
Also, have your drum brakes checked.