If all you want is a UK patent, then the process is fairly straightforward. Once a draft patent has been prepared then the document is filed with the UK Intellectual Property Office (IPO), along with the application form (Form 1), and the filing fee. Before the application can be granted the IPO will conduct a search and an examination, to check whether the invention is suitably novel and inventive. A search fee and an exam fee are payable, but these are relatively modest fees.
Eighteen months after the application is made, the application will be published. At this stage, the search has usually been done, and the applicant will have a good idea of the chances of getting a granted patent. If an early examination has not been requested, then, following publication, the examination will take place. At this stage the patent examiner will flag up any issues they see with the application. They may, for example, argue that the invention is not novel or inventive, due to it having been described in some previous document. The applicant may argue this point, or may amend the claims to attempt to get around the citations. This can be tricky, due to the limitations on adding any further material to the application - you can only work with what is in your application. The skill of the patent attorney is invaluable at this stage.
If you successfully argue for the novelty and inventiveness of your application then the IPO will grant the patent - typically 2 to 4 years after the filing date. At that stage you have the ability to stop others using your invention, as it's defined in the granted claims. To keep your granted patent in force annuities must be paid to the IPO. These start off fairly low (around £50/year), but as the patent gets older they do increase. The patent can last a total of twenty years from the date of application, after which the patent will die and the invention will become available to all.
Obtaining a patent in one or more foreign countries is a more involved process, with various routes and options being available that will be chosen based upon budget, desired speed of grant etc. One popular route is to file an "International application" using the Patent Co-operation Treaty (PCT). With a single application, the PCT covers most countries in the world, for a period of up to around 2.5 years from the first filing date. Following this period the applicant must choose where to continue, and pursue the application in the chosen countries or regions, but the 2.5 year single PCT application is often used to gauge the merit of the invention, delaying the individual country filing, translation and attorney costs.
Other approaches may be used. For example, you can file directly in the countries of your choosing. This can lead to a quicker grant, but all translations will need to be provided from the start, and that period at the start of the process where you can evaluate and develop your invention before incurring this expenditure is shortened.