The spread betting provider makes the prices you see on your trading platform.
“Is it synchronised with the exchange?” -> Not perfectly, no, but the spreadbet will take the market price and it’s own pricing will be based on that and will follow the underlying prices very closely.
For shares, spread betting providers take their from the underlying markets and then add a fixed mark-up to the bid-offer spread. The process is somehow different for forex as there is no central exchange. For instance, as a provider of an over-the-counter currency product, GFT has access to and trades with all the major banks. The best bid and offers from this process are fed into a trading platform and form the basis of the company’s spot rate spread betting quotes.
In my experience (forex trading), the difference is rarely more than just a a pip difference on major currency pairs. Around news times things can get a bit different and sometimes the difference is much larger. You should be aware though, that if at a particular time they are offering something far different from the real market, a trader could arbitrage, and so the spread betting companies don’t really want the price to get too far away for very long.
If you want to test this out, try registering for a demo account with a broker and see for yourself…