I don't see its much of a decision Ewan. Even if your profits are 30k, surely your mad to be a sole trader. Your giving money away to the tax man if you stay as a sole trader.
If your an old boy about to wind down, I understand. Otherwise, its black and white if you ask me.
How?
30k equals a saving of £1673 if you are limited over a sole trader. Subtract the extra cost of being limited, which is £675 for my accountant, so I will use that figure. If we use the figure of £95 for a sole trader, which is extremely cheap. 675-95 = 580 difference. 1673-580 = £1093 saving of being limited compared with sole trader at a profit of 30k.
I used to do my own as a sole trader, but you need to really be on top of what you can claim. Bottom line, if your profit is 30k and your limited as opposed to being a sole trader you will save approximately a grand per year. Simple as that really.
There are other minor differences between the 2 company types. Minor pros and cons that you can google if your curious. Lets face it, money talks. Thats the "how" of why I think its black and white.