I disagree. There are 223 places available for History at Oxford, and just 13 for History of Art.
Imo, you really don’t need a service: the UK application system is much more straightforward than the US system, and there is more than enough info available free online. There are only 2 variables: your Letter of Reference (from your school) and your Personal Statement (why you and the course you are applying to are a great fit). On both the UCAS and Oxford websites you can find lots of info on how to write both (give the info on the LoR to your GC or the teacher who will be writing the LoR). There are some great CC posters with plenty of experience at the UK unis who will be happy to answer questions.
Be aware that Oxford will see all of your scores, and your (very relevant) low History AP scores are unlikely to be offset by high subject test score: In the UK APs are viewed as more rigorous (particularly for an essay-based subject), as Subject tests are all multiple choice- a format the English don’t particularly rate. Also remember that whoever writes your LoR will have to “predict” the scores for any standardized testing that you are taking as a Senior, and the UK unis can choose any combination of achieved and predicted scores they want to be the basis of any offer they make. An “unconditional” offer is just that- you’re in. A “conditional” offer means you need to provide official score reports to show that you have met the offer (whether with test results you already have, or once you have taken the exam).