In short, don't do it.
There are few advantages with being a business seller. You will receive a small discount on FVFs but it's only small and the criteria to be awarded those discounts are utterly draconian. You're at the mercy of the Royal Mail and couriers as well as dishonest buyers. Only takes a couple of tossers and you miss your target and don't get your discount.
The benefits come when you have a "shop" as a business seller. This costs you a minimum of £20 a month but does give you a load of free listings and allows you to keep things listed constantly until it sells. This in itself is not as great as it sounds though, but that's a whole separate topic.
The trade off for the relatively poor benefits of an ebay shop mean you will no longer receive the £1 final value fee or a % off FVF promotions amongst others. You'll get ZERO promotional discount deals. They have saved me well over £1,000 in the last year and I'm not about to give those up.
Also, don't think that just because you've paid tax that you won't be liable again. If you look at HMRC's definition of when you are considered a trader or when you should pay tax on items you sell, although there's no specific numbers it does rather sound that selling more than a handful of bits each month makes you liable. I know some countries have strict definitions. I forget which EU country it is but I do recall that if you sell more than 2 things a month you owe tax!
Also as a trader you MUST offer returns. In fact the requirement is over and above Distance Selling Regultions. Nuff said, really.
Anyway, I could waffle on for pages about the ins and outs of ebay selling but the above should serve enough to put you off becoming a business seller. In short: DON'T.