There are several possible reasons including:
Avant have a select list of brokers they use, make sure whoever you have been contacting is on the list
Is there something they know that we don't about Avant?
Assuming you have spoken to a broker on the Avant list, then yes, the brokers know that Avant have been very selective and risk averse in the properties and customers they accept. From what you have told them they may feel you don't meet Avant's criteria
You say you are new to the mortgage market "here". There have been a few examples on AAM where people are UK nationals or spent time in UK and ended up having a lot of difficulty with credit reports and paperwork that in the end it wasn't worth the hassle. The brokers you spoke to might have the same view
15 years to retirement so options limited on length of repayment term. LTV is in the <60% category.
Your LTV may be good but you don't mention your LTI. If you are looking to borrow a lot relative to your income then the brokers may already know that Avant won't entertain your application. For example an LTI of 3 for a 50yr old is a lot riskier (to Avant) than a 30yr old. So again, it is possible that the brokers are ruling out Avant because they know your application would be rejected
And then there are all the other factors that could make you look like a riskier customer to them which could include being self employed, being a single income family, having several dependents which at your age are potentially looking at expensive 3rd level education or maybe you are planning to live in an rural area that Avant won't approve.
There are many potential reasons that a broker wouldn't recommend Avant to you and if you have a sense of urgency to buy, they may feel it is better to go elsewhere to get AIP quickly