Business Expenses prior to initial set up of business (UK)

I'm setting myself up as a sole-trader in the UK, as a Life Coach. I hope to put out my first advertising and get my first paying clients over the course of the next month, or so.

It's clear to me that I can claim some costs as business expenses prior to actually doing business.

E.g.

• Buying Domain Name
• Setting up website
• Initial advertising
• Buying a business phone
• etc. etc.

These are all things that specifically relate to setting up my business.

As I trade I will buy books relevant to my trade to continue learning and keeping abreast of my industry.

It's clear to me that I should claim those books as business expenses too.

But I've been buying books about the industry as I headed towards my free-lancing, for the last several months - Can I claim those too?

A month ago, I bought a book about setting up a company, specifically to aid with this process. Can I claim that as an expense?

I have just (in the last 3 months) completed a 2-year Masters in Positive Psychology, which is my relevant qualification for doing this. If I'd done that whilst I was already a sole-trader, I would expect to treat this as an expense. But I actually did it before being a sole-trader. Can I claim that as a business expense?

More generally, how do I work out what I am and am not allowed to claim? Does HMRC have a guide for this?

Supposing I AM allowed to claim this, then how do I do it? Do I have to claim it in the first financial year I trade? (Which would be a shame since I likely won't earn enough to pay tax before April, in which case the expenses are irrelevant) Or am I allowed to back-claim it in the 18/19 financial year?