You can rest assured - Bitlocker is exceptionally secure. As stated, using brute force to crack the encryption key is unfeasible given the extremely high number of combinations - 8 sets of 6 digits with possible values of 0 - 65,536. The use of TPM prevents any pre-boot tampering, via BIOS, alternate operating systems, slaving of disks to other systems, and including the Windows 7 + F12 scenario you suggested.
The only possible flaw with Bitlocker is that the password is stored in memory whilst the machine is booted and in hibernation. There have been reports of the password being stolen by performing memory dumps from machines booted from a hibernated state, but this requires very specific skills and software. You can easily circumvent this risk by not using hibernation.