"VAT is mostly done through self assessment. so Yes, it is that easy"
Theoretically.
In practice, there are various red flags built into the system that will get you onto their radar.
One of the red flags works in the following simplified manner :
In a normal business the expectation is that you will always end up paying HMRC something, because the expectation is that a business is profit-making and therefore the balance always tips in HMRC's favour of being the one to receive a cheque in the post.
There are perfectly kosher circumstances whereby a business may occasionally end up with a HMRC sending them the cheque instead. However, too many of those circumstances (or too odd a pattern or whatever), and at best you'll end up with a friendly tea & biscuits session from your local HMRC officer who will come for an informal sniff-around your offices and your files ... but if you're doing something really fishy, then the big guns at HQ will take interest.
There are other red flags out there too, some of them industry specific. There are various "schemes" that HMRC are aware of in relation to IT and Telecoms, and they take specific additional monitoring steps for companies involved in those areas.