By-passer, normally this topic is better supported by IG technical support, even in IG forum.
In any case, you can find the info from here, https://labs.ig.com/rest-trading-api-reference/service-detail?id=553
For account leverage, I think you mean margin used? There is account information, you can obtain account.balance object for related information according to the table in the page
In case of instrument leverage, you can see the info here to obtain market.instrument.marginFactor, https://labs.ig.com/rest-trading-api-reference/service-detail?id=528
You can play around with their API companion, https://labs.ig.com/sample-apps/api-companion/index.html