Here’s our article on heating options for your van: https://ukvanlife.co/campervan-heating/ it covers the main options you have and the pros and cons of each. Personally I’m (and many others are) a fan of the cheap diesel heaters because they are efficient and value for money. Here’s the one most of us use: https://ukvanlife.co/diesel-heater/. They are relatively easy to install and use even by the most untechnically minded so don't be put off by the thought of installation.