A trip to Peru has the advantage of being simple to prepare for. There is no need to do any complicated organisation before you leave. Peru is a tourist destination that has decided to make life easy for visitors.
To take a trip to Peru nothing could be simpler. A current passport is all you need. British citizens have no need to obtain a visa.
Regarding vaccinations, none are obligatory. Your international vaccination records will not be requested. It's therefore not necessary to bring them with you. On the other hand you are strongly advised to have vaccinations for yellow fever, typhoid and hepatitis A. Malaria prevention treatment should also be taken if you are intending to go into the Amazon forest.
Before leaving, think about taking out travel insurance. This is even more important if you are thinking of going trekking.
Finally, if you intend to drive whilst there and to hire a vehicle, you will need an international driving permit. Some car hire companies insist on them. They car easily obtained before you leave from the Post Office or by post.
Your plane will touch down on the tarmac at the Jorge Chavez International Airport in Lima. You will have arrived in Peru. The border police will authorise you to stay for up to 2 months. If you ask politely and don't insist too strongly you can request the right to stay for 3 or 6 months.
Once there, if you want to prolong your stay, you just need to leave the country, even by road, and to come back again. You will thus obtain a new authorisation to stay for 2 more months. The other option is to simply stay in the country and resign yourself to paying 1$ for each day that you overstay. In the end it's not too expensive, just symbolic and most of all easier and less hassle.
Above all, make sure that you don't lose your immigration card, which is given to you as you enter the country. You will be asked for it when you leave. In case you lose it, it is always possible to have a new one made at Migración but you waste valuable time on paperwork and you will have to pay a fine of about £5.