Miley, 23, clocked a time of 4:35.67 minutes in winning the 400m final on Thursday, going more than five seconds under the qualifying standard required for Barcelona.
And despite following that up by qualifying for the semi-finals of the 200m discipline on Friday morning in Sheffield with a time of 2:16.91 in the heats, Miley insists the pressure has been switched to the likes of 17-year-old Siobhan-Marie O'Connor to make the squad.
"It's getting harder every year, there's a lot of youngsters coming through nipping on my heels," said Miley.
"I do safely say the 200 medley is pretty much going to be Siobhan or Sophie (Smith), but I'm not going down without a fight, but I think that title's probably going to be gone.
"The 400 medley I've been really chuffed with. It's kind of crazy but it's been a nice progression seeing it and just seeing the field changing every year."
Liam Tancock bounced back from the disappointment of losing the 100m backstroke final on Thursday by cruising through to the semi-finals of his preferred 50m event the very next day in a time of 25.32 seconds.
Tancock's four-year reign as British champion in the 100m was ended by Chris Walker-Hebborn, who produced the fourth fastest time in the world this year with 53.38, but Tancock will be gunning for a time lower than 25.27 to all-but secure a ticket to Barcelona later.
Meanwhile, newly-crowned 200m butterfly champion Jemma Lowe will be looking to make it a double in the 100m and got off to the perfect start in the heats.
The 23-year-old saw off the challenge of Sophie Allen in the 200m and breezed home in a time of 58.64 in the 100m, with the semi-finals to come Friday evening.
Elsewhere, after beating Mark Foster's British record in the 50m freestyle on Wednesday, Adam Brown swam 24.11 in the heats of the 50m butterfly but opted to pull out of the semi-finals.
- Sports & Recreation