I know, it's very confusing. The shipping industry can get quite incestuous as to who is linked to whom (I know, I work in it).
Basically the people behind aferry.to are the same people behind HD Ferries and the Portsmouth operation of LD Lines. They also provide the booking engine for LD Lines and Transmanche. Their booking system is used by other operators too - at one time (maybe still) including P&O.
So prices should be the same as booking direct, at least most of the time. Sometimes there can be a bit of a lag with changes to fares, and sometimes special offer fares will be routed through the booking agents (and at other times will only be available when booking direct). So to be sure, you should always try a quote from the operators themselves if you want to be assured of getting the best deal.
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