Hi Michael,
Welcome to the Forum; if you select the Handy Tools link at the top of the page, you'll get a link to the Expat guide, this describes the process you'll have to go through to get into the UK. I'd double-check this job offer, as "volunteer" is not on the UK list of "", so unless you have a UK passport, you may struggle to enter the country.
To answer your specific question, you will find racism everywhere in the world; having said that, the UK is generally multi-cultural and the risk of specific racism is small; my own opinion of the UK is that you're more likely to come across xenophobia than blatant racism.