You need to ask yourself whether you could end up in a position whereby the business has massive debts which you won't be able to service.
As an example, my friend was running a plumbing business and accepted a big contract from a customer. He paid out loads of money for materials, knowing that he could pass the costs onto the customer in his final invoice for the work. However the customer had financial difficulties and declared himself bankrupt, leaving my friend with loads of debts and no means of paying them. That, in turn, forced my friend into bankruptcy himself. (Fortunately he was only renting his home. If he'd been a home-owner he'd have lost his home through the bankruptcy).
If my friend had been running his business as a limited company he would have been able to put it into liquidation, with no affect whatsoever upon his personal finances.
So, if a problem with your business could leave you facing personal bankruptcy if you operated as as a sole trader, you definitely NEED to operate the business as a limited company.
However if such a situation couldn't occur (which seems unlikely when providing a service which doesn't require large purchases of supplies) then operating as a limited company just seems to be to be adding unnecessary complications to the way you'll be working (such as submitting properly audited annual accounts to Companies House).
Another example of the complications of running a limited company is that you have to submit BOTH 'employer' and 'employee' figures to HMRC. i.e. as a director, you'll be an employee of the company, so you'll have to pay PAYE. However as the owner of the business you'll also be responsible for the employer's side of PAYE (and National Insurance) arrangements, which means a lot of form-filling. (Written from experience!).
As I see it, the only type of 'registration' you need is with HMRC as operating a business (on a self-employed basis). That's totally different to registering a limited company at Companies House (which I don't think you needed to do anyway). You should note however that HMRC won't allow you to register a (self-employed) business until you're ALREADY trading; you can't do it in advance.