Buy BritRail Pass before working in the UK

I'm going to work in the UK for two years. But I'm now a non-UK resident. Am I eligible to buy BritRail Pass initially before entering UK? I would be considered a UK resident if I have been living in the UK for more than 6 months. Does this mean I can buy BritRail Pass for my first 6 months before I become a UK resident?
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