I have used a patio cleaner obtained from B & Q for cleaning the grime of patio paving slabs. They are 30 years old and were previously pressure washed, which unfortunately took off some of the original protective surface which now allows them to get dirty and covered in lichen more quickly. However, if used with plenty of water, lots of elbow grease and a stiff bristle brush they can make a considerable improvement in appearance.
As for killing weeds under your gravel, you can buy Verdone or some other liquid weed killer from a garden centre. Dilute as per instructions (usually one lid full of liquid to one gallon of water) and pour over the gravel using a rosette spray on your watering can. Preferably do this just after rain so that the liquid sinks in better into the gravel. If you spray it on when the sun is shining it may evaporate too quickly before it's had a chance to do its job.