I'm not a lawyer, but it's been my experience that if your invention combines A and B, and the examiner can find A and B in separate patents, then you're pretty much cooked. If you find both A and B in a single patent, then you'd better hope the patent is expired. If you find prior art, then your options are to give up or narrow your claims. That can be a gut wrenching, if you're already in the hole to a lawyer when you have to make this decision. At best, narrowing your claims still lets you protect the important elements of your invention.
If you find a "close" patent, then following its citations, and the patents that cite it, is a good way to chase down more possible prior art.
Now, I've found that it's touch and go whether the examiner will look outside the patent literature. As it gets easier and easier to search for anything that's been written about anything, fertile ground for finding prior art will of course expand. It could include the academic literature, so if you're a student, you might be able to get some (free) help from your friendly neighborhood reference librarian.
Eg, https://register.epo.org/application?number=EP13189587 has BWT as their representative.
If you find the quoted prices too high and your offers of lower amounts are rebuffed then ask again here stating the offers you were given.
Ultimately you can submit a complete disclosure of your invention and then mould the actual application later so if you're doing this as a prelude to seeking funding then that is probably the cheapest way to go. A good patent attorney is likely to be able to get you a relatively useful patent that is actually more broad than your actual invention however.
https://oedci.uspto.gov/OEDCI/ might be useful, it's a USPTO list of those registered to practice with the USPTO, you can query by state, for example.
If you're after a UK agent feel free to PM me for further info.