I don't know much about thrift or second hand stores in the UK, but that's one place I do shop regularly for some of my soapmaking things. I have found silicone molds, wooden boxes that are well suited for soap molds, pitchers suited to mixing my lye solution, bowls for mixing soap batter, a used crock pot/slow cooker, etc.
Other than that, finding a source in the UK for your caustic soda will be important. A stick blender is not absolutely required, but certainly quite useful for soapmaking, especially if you use slow-to-trace formulas.
Read up on the types of plastic that are safe to use with lye solutions before you buy. Read up on how to extend the life of your stick blender when used for making soap (very important before you first use a SB in soapmaking). Read up on proper safety precautions and gear for soapmaking before you make your first batch. The Beginner's forum here has lot good information. Start with the 'sticky' posts in Beginners and Lye-Based sub-forums.
What NOT to buy: pH strips - waste of money for lye soap as they are not at all accurate and don't tell you is a soap is safe. Learn to do the zap test properly (no equipment required) and that's all you need to do to find out if your soap is safe to use.
Expensive online training courses! Don't pay for expensive online training courses. There are so many available as has been mentioned by others, that will teach you the basics. Ask questions here at SMF and learn from other's experience for free.
I have never, ever, never in my life, asked a soapmaker or manufacturer if they had a diploma or where they learned to make soap. I am an old woman, so that's a long time of not caring about a diploma for soapmakers. I have asked questions about how they make their soap and what's on their soap labels. Their answers will tell me if they know what they are doing and if I am satisfied, then I may buy their soap. If they can't explain a few basics of soapmaking or why something is on the label, they could display a diploma all they want, but I would not buy their soap.