I'm assuming that unlike a book blurb, you do include details of the ending?
That kind of depends -
personally I go up to the Black Moment and the moral choice that comes at the climax: Our Noble Hero can do X, but his wife dies or do Y, and she lives but all the unicorns are lost forever...So, further than a book blurb, but not so far I reveal what choice Our Hero actually makes.
All the (US) style query needs to do is make someone (an agent) read
your book, as opposed to all the other books they've been pitched this week (depending on the agent, this could be up to 100 other books. So it's important to get it right!). Use everything at your disposal, and leave out what won't help -- this is part of the craft of writing, choosing which is which! Like I say, read lots of queries, see what works, what doesn't. Also research agents! Some state they MUST see the ending in a query (so give it to them) some don't (go with the version you think works best)
Ofc, with a UK query, you send a synopsis normally, so giving the ending isn't a worry. Then again, the blurb/pitch/synop part of your letter is often shorter (Note, I have had zilch success with UK agents so take that for what it's worth!)
You'll want to research if they are legit as well (I recc AW's Bewares forum and Preditors and Editors, run by Writer Beware). Even if they aren't scammers, if they aren't making sales, or are only making sales to micropresses that take unagented subs welll...a bad agent is worse than no agent
Another tip is to send your query out in batches of, say, 5 or 10. If you get no bites whatsoever, tweak your query.