I've not bought properties thru' any of the companies you name. I have heard of assetz? is that whom you meant? Not bought any from those you mention.
However, I've bought of a couple of properties through these so called property sourcing companies and IMHO yes, you will be able to buy with less deposit than originally required if you went all alone but theres a price to pay for this, many will be over-priced new builds (often 2 Bed Flats) when you complete the sale you will be completing with several other investors; the net result will be a lowering in the re-sale value as so many new builds come onto the market and you will be in competition with the others to get it rented out, the end result being you will often have to rent it out at less rent meaning lower income for yourself. However, if you buy in established property there is more chance of a better re-sale value and it's chances of it being Let out quicker and at better rent will be far greater.

There's a couple of property sourcing companies that I may consider but have not used them yet as they seem to either source individual properties or have no finders fees and offer almost the same deal (and property) as those with Finders fees. Overall though it's better to source your own properties and do your own due diligence as no one will have your best interests at heart than yourself. Good Luck!