Confused about OPKs and preg test after miscarriage

We’ve just had a miscarriage at 6 weeks. All happened very fast and today my hcg was only 25. The hospital initially said we could try again as soon as the bleeding stopped and that we are more fertile now, and I figured we’d use opk sticks and bbt to check ovulation to help with dates. But then they told me to wait three weeks to do a pregnancy test - my cycles were only 24 days so that is too long for me, I’ll most likely have missed the window. But I know opk tests can pick up hcg too. Help! I’m thinking I’ll do the pregnancy test in a week and try as soon as we’re ready? (I’m in the U.K. so more bloods is not an option)