I am heavily involved in the cleaning industry and also run a premium service level private cleaning business.
If you are using an agency then the cleaners will be self employed and therefore not entitled to sick and holiday pay. The agency will however provide insurance but in a lot of cases they shirk this liability saying that as the cleaner is self-employed it is not their liability for damage she caused etc! this is why the agencies can be as cheap as a self-employed individual cleaners, they are more of a match-making service provider than a fully-managed private company.
Agencies are cheap for sure but they do not provide the level of service you will get fro a more expensive fully-managed service. it just depends what your priorities are in terms of service level.
There are so many service models out there to suit all different requirements and all different budgets, it depends on what you consider the most important things to you, for some this is quality of cleaning and customer care for others is is a quick reliable 'do' which doesn't cost a lot.
There are also the 'cash-in-hand' brigade who sometimes charge as little as £8-9 per hour I believe but you need to steer clear from these types, I have seen so many homes damaged by such and it is not a legal arrangement, it is not worth the risk to save a few pounds. I would not include this in the list of 'services' they do not offer customer service at all and very often dissappear on you when they feel like. (or HMRC start investigating!)
The large franchise is the most expensive model (around £25 per hour) as it will need to be to fund the huge business machine from which it operates and although the actual cleaning delivered is so-so they are completely reliable and managed and you could do worse (especially with the cash-in-hand cleaning lady types!!). I have to mention that I do believe the cleaners get minimum wage with this model too so bear that in mind.
Independent companies like mine are nationwide and meet the gap in between the big franchise and the cheap agency, although all different models we all tend to be fully-managed supplying all equipment and materials and taking full liability for what happens in your home, staff are employed and fully trained to use the chemicals we supply and health and safety is taken care of. We are not cheap like the agencies though but not as expensive than the franchises. It is a more personal service than either and relationships with our clients are very warm.
I am not touting for business (as someone once suggested) as our teams are always fully booked - having to start a waiting list for our services again now until I find new staff which takes blooming ages! (sigh)
I just hope this is a helpful post by someone on the inside of the industry, just to explain how it all works so you can make up your own mind which is the best value for yourself and your own needs.