You can go to most postal service companies, whether they are owned by the government or in partnership with them, or even a mailing service by itself and mail out just about anything that you want. There are always restrictions on items that are not allowed overseas or in other places of the country. As there will always be laws to follow as well when it comes to perishable food, weapons and other items that could cause harm to either the carrier or another the person on the receiving end of the parcel.
A courier company in London may have similar restrictions that will be based on weight and size of the parcel that you want delivered. Typically, they are resigned to small documents and can go as large as blueprints, maps and drawings that are found in roll tubes. Just about every envelope size is accepted by a courier service, as that is the prime type of delivery that a courier is in for. Small packages are accepted up to a certain size depending on the company that you are using. Typically, something that can fit in a backpack or on a bike and sometimes accepting those large enough to carry a few books.
Weight can be an issue so even a small parcel that weighs too much could be turned down. A courier may not necessarily be driving the package in a car or van as they may be on a bicycle, motorcycle, moped or scooter. The smaller transit couriers are faster and more frequent, but they cannot carry a lot in the packs or the bags located on their transports.
Courier services can deliver anything you can think of that can legally be transported by the government and county that you are in. Medications can even be delivered by a courier service so long as it is signed for and legal for the medication that is being transported to leave the pharmacy or hospital.