Here is a simple how to on encrypting the drive with a pre-boot key using Bitlocker on the Surface Pro 2 when it's not joined to the domain.
1) Once you are logged into the Surface, open the command line as administrator (this can be done by searching for cmd and then right clicking it and selecting run as administrator or by right clicking on the start menu and selecting Command Prompt Admin).
2) Type gpedit.msc and hit enter.
4) Double click on Require Additional Authentication at Startup and select enabled. The trick here is that on the left side you have several options. You can not allow some and require one. So set them all to allow.
5) Select Enable use of BitLocker authentication requiring preboot keyboard input on slates and set it to enable.
6) If you want your users to be able to use a password instead of numeric pins Select Allow enhanced PINS for startup and set it to enabled. The rest of the settings are Administrator preference.
7) Once those settings are complete, go to the search tool and type bitlocker (or alternatively open up windows explorer, click on the C drive and use the Manage tools, select BitLocker and turn on Bitlocker from there) and select the BitLocker Management utility.
8) Turn on Bitlocker, and leave the check box that allows it to check the file system and go through the wizard. Select Use PIN, and enter in your desired PIN that meets the length and complexity requirements you set in the GP.
9) Choose where you want to store the BitLocker Key. You can tie it to your Microsoft Account, or save it as a file. I chose to save it as a file on my backup drive. I will also set it on our network so that it's backed up as well.
10) Don't allow BitLocker to restart the system, instead select restart later, and shut the system down.
11) Once the system is powered down, press the Volume Down rocker, hold it and power the system back up. Once you see the Surface splash screen, you can release the power button. Once the BitLocker password screen shows up, release the Volume Down rocker and enter your password. If you don't hold the volume down rocker you will not be able to use any keyboard to enter in the password.
12) After the system boots, you will be able to log in and keep working while BitLocker encrypts the C drive.