OP are you UK based? If so you can do much of the research yourself.
If you are abroad do bear in mind that the majority of agents will only recommend schools from whom they receive a fee. That is regardless of whether the school is the best fit for your child. When we lived abroad we were astounded by the advice and placements some families accepted eg overseas boarders taking places ar schools in which all the other students went home on a Saturday night eg very clever overseas children taking places at schools where the CE requirement was 55% and they had nomacademic peer group eg soccer mad children taking places at schools which only played rugby.
Those services are usually for overseas parents who haven't a clue and don't know where to start.
If that's not you, I advice getting a Good Schools Guide subscription, drawing up a list of those within the distance you're happy to drive for exeats, half-term etc, and then search those schools on Mumsnet for more unofficial info than you'll find on the Good Schools Guid.e
I found that the Independent Schools Show in London was the best way to find out about a wide range of schools. There were lots of good schools there and you have a chance to talk directly to the admissions tutors (although not all). But this gives you a chance to narrow down some options and there are less then that you need to contact directly.
I am not sure of you have just missed the latest show or whether there is one coming up imminently but do check their website. If you register in advance, entrance is free.
I am not sure I agree with previous posters. A good consultant will get to know a parents/child's needs (even the needs a parent does not know they have) and have an idea of which schools will both fit, and be likely to accept. And what preparation will be required.
If nothing else this is a huge investment decision. Why not pay for good advice? (As well as doing your own research.)
Some kids will fit happily into most schools but not all. A mistake when choosing a boarding school is a huge mistake.
Part of the problem with MN is that people will use to promote their own views or justify their own decisions. Get a good short list first, which in turn will help clarify your priorities and then as for other people's experience/views. Then filter in the light of your own needs.