I think trade body costs would stay the same. I can't imagine insurance costs being hugely different. One thing to watch out for though is van insurance. If the van(s) become(s) property of your limited company then it would be normal to register it in the company's name for insurance purposes. The company will have 0 years no claims so expect a hit on your van insurance for the first couple of years. In addition, you yourself will stop accruing no claims which may be a problem if you don't also drive a car.
This was a bit of a shock to me.
Other than that, limited companies have another tax return to complete each year which your accountant will probably charge you more for.
If you currently earn enough profit as a sole trader to take you into the 40% tax band then you will definitely be better off as a Limited Company. You won't pay more than 20% tax on drawings from a limited company. Basically the plan is you take £10k per year out as a wage (taxed at 0% with a bit of NI to pay @ 12.8% ish, employers NI is covered by HMRC up to 2K per year). After this you pay yourself share dividends on which the company has already paid corporation tax @ 20% as they come out of the profits. As an individual you pay no tax on these up to the higher tax threshold. A £40k wage/dividends package from a Limited Company is worth around £60k as a sole trader (once the sole trader pays their income tax and NI).
Hope this makes sense.
As already mentioned, the Limited Company is a separate legal entity and it will start with no credit history. Is this really an issue though? All of my customers pay for their sanitaryware/tiles etc up front so I don't really have any need for credit. If you deal with businesses this may be more of an issue but not for domestic work.
If you find your business growing to the stage expansion is necessary then a Limited Company (in my opinion) provides a much better framework to do that, especially when you start adding multiple drivers onto multiple vans for insurance purposes.