Rjan:
Bear in mind that there’s a big difference between legitimate expenses and fabricated ones. The taxman is unlikely to accept that an agency HGV driver found it necessary to wine and dine whilst hammering out a “business” deal.
You can fabricate expenses until the cows come home. The question in the end is always whether a tax investigation occurs - if it doesn’t, then you can claim for the expense of the moon on a stick without anybody telling you different, and if it does occur then you’re going to have a significant bill to pay which could mean a new mortgage.
Apologies if I inferred use of fabricated expenses, I didn’t mean it to be read that way. All expenses would have to be interpreted as legitimate. Depending on how you ran the company, you could legitimately incur many types of expenses, including wining and dining potentially depending on what sort of work you were looking for.
For example (And only as an example) if you were doing 90% agency work with an agency and spending the other 10% of time picking up odd jobs here and there, for example moving a track car from point A to point B, perhaps even hiring a suitable vehicle to carry out this job, then meeting with the client over a drink to discuss the finer details is perfectly legitimate and could be seen by some as the wining and dining.
The above example would likely go towards keeping you outside of IR35 regs and so you could claim your mileage expenses, utilities etc.
If you ran the company doing 100% agency work with only 1 agency, then yes you are likely to fall within IR35 regs and you could be seen as being an employee without the protections afforded to one and then HMRC might go looking deeper into you.
Rjan:
“Small business” 
You mean how friendly they are to tax evaders?
One of the biggest things to bear in mind with Ltd is simply all the hassle involved. Are you being paid for the time and effort of keeping receipts? Paid for the time and effort of dealing with an accountant and researching tax rules?
If you outsource all the hassles back to the agency, who print your invoices for you etc., then you only impugn the very notion that you’re running a legitimate business.
I always quote an old rule of thumb which is that if you’re (genuinely) working for yourself, you should be charging 3 times (300%) the hourly rate of an employee.
If you’re charging some ridiculous fraction, like 10% over the employed rate, then you’re just shafting yourself, and even a tax fiddle won’t convert a 10% premium into 300%.
A lot of fellas used to fiddle because they were dodging the CSA, or they’re massively understating their income (or overstating their hours for a given income) in order to claim benefits, but the CSA is not something that applies to most, and the benefit rules have been significantly tightened so that going forward that is unlikely to be an advantage.
If you don’t like the term “Small Business” Do you have a preference ?
There is a big difference between tax evasion and tax efficiency - Typically time in front of a judge and a big fine 
Ofcourse there is no issue if you feel you aren’t paying enough tax and wish to pay more, but don’t force that on others who are already paying enough in the eyes of HMRC rules
Depending on the sector, such as engineering, that 300% rule isn’t a million miles out. For logistics, especially drivers that rule might not be so realistic sadly. I do agree there needs to be much more than a 10% markup to make being a Ltd Company work and you do need to run the numbers yourself to make sure it does work out in your favour.
How a company director chooses to charge for his time for the running of the company is between him, his accountant and the director as an employee - I’m not sure why any agency would be involved with that.
Above all else, find a decent Chartered Accountant to guide you as it can be easy to come unstuck and incur fines - The Chartered bit really does seem to make a difference when it comes to HMRC investigations, or lack of if a Chartered Accountant has signed off on the numbers for you.