It's difficult to give advice without some basic knowledge of your general personal circumstances, such as how long you want to rent or lease a property, how large, where you are now and how large your family is.
If you're not English and aren't located in the UK then you must be more cautious. If you're a student coming to the UK and/or prepared to accept basic accomodation at the outset, then you could stay at a YMCA and look for more permanent accomodation when you get here. Universities often also offer reasonable accomodation during the summer holidays. Probably not the best choice if you're visitng the UK with your family on a two week holiday but it could be ideal for a backpacker on a budget holiday wanting accomodation located in central London.
If you're after something more permanent then if you visit this site
http://www.arla.co.uk/and use their facility to seek out their members, then you're going to go a long way to avoiding outright scammers totally, and it appears they operate to a high standard, but they are probably for a more lengthy arrangement.
You're better off seeking out companies that are members of reputable organisations, but don't just accept websites with fancy logos copied by scammers. Another member here was seeking out cheap flights and using a search site which kept throwing up very dubious deals, but that's the risk of using cost as the sole criteria of your search. Scammers can always offer the cheapest deals.