Transferring money from US to UK

I am a Uk citizen but a US resident, but not citizen. I would like to cash out my company stock (RSUs) which I would have to pay Us taxes on any earnings on in the US, but then want to transfer that entire lump sum to the Uk, Do I need to pay any taxes on it? Does it make a difference if I do it before or after I actually move back to uk and become a uk resident and tax payer again? Does the amount matter? It would be ~500,000 gbp

Hi L woo,
This depends on your actual tax residence status for the year.
If you qualify for split year then you only report any foreign income for the UK part of the year RDRM12000 - Residence: The SRT: Split year treatment.
If you do not qualify then you will need to report all your foreign income to the UK Tax on foreign income.  
The guidance at RDRM12150 will help you work out if split year treatment applies. 
Thank you.