BBC One is more aggressive with its scheduling than commercial channels these days so that's no surprise!
Hardly, you might have a point if they were flinging the two existing shows off despite being billed in the TV guides, inconveniencing viewers, and you may have a point if the shows they were flinging off were part of a continuing series where their loyal audience would be annoyed if they were removed before they reached their conclusion. But they're not, they're two LE shows which can be watched in any order at any time and which are getting low ratings and AI figures.
The replacement will be billed in advance, so as far as most viewers are concerned, Secret Service and Animal Antics have finished, and it's certainly their prerogative to rearrange their schedule if it's not working.
Interesting that Britain's Brightest actually gets shorter as it goes on. Not really sure they needed the extra fifteen minutes.