You must register for VAT when your taxable turnover exceeds £90,000 in any rolling 12-month period, or when you expect it to exceed that in the next 30 days alone.
'Rolling' is the word people miss. It's not your accounting year — it's any 12 consecutive months. Check monthly if you're getting close.
Voluntary registration can make sense below the threshold if your customers are mostly VAT-registered businesses. You reclaim VAT on your costs and your invoicing stays clean.
Once registered, you charge VAT on your sales, reclaim VAT on your purchases, and file returns quarterly (or monthly). Under MTD, all of this happens through compatible software.
The Flat Rate Scheme can simplify VAT for smaller businesses under £150,000 turnover — you pay a fixed percentage of gross turnover instead of tracking VAT on every purchase.