From HMRC:
https://www.gov.uk/working-for-yourself/what-counts-as-self-employed"Selling goods or services
You could be classed as a trader if you sell goods or services. If you’re trading, you’re self-employed.
What counts as trading
You’re likely to be trading if you:
sell regularly to make a profit
make items to sell for profit
sell online, at car boot sales or through classified adverts on a regular basis
earn commission from selling goods for other people
are paid for a service you provide
What doesn’t count as trading
You’re probably not trading if you sell some unwanted items occasionally or you don’t plan to make a profit. You can’t use any losses you make as part of a hobby to reduce your tax bill.
Contact HMRC for advice if you’re not sure whether you’re trading."
Whether you register as self-employed or register a company depends on a range of factors including things like whether you want to separate liability or use losses to offset tax elsewhere (although there are limits to this). Likewise the VAT decision depends on other things - largely whether you anticipate making sufficient purchases so that the VAT saving would be worthwhile.
Also important to note that as self-employed you also have National Insurance contributions to be made.