I have roughly around 120k to move from my own personal account in the UK to my own personal account in the US. What are the tax implications of this?
There are no tax implication for the transfer of personal funds from one personal account to another personal account. Fund transfers are not taxable income and are not reported on your personal tax return.
Presuming you have all ready paid tax on the money in the UK, there is no income tax ramifications on a transfer.
I hope that on that sum, you have been filing all of the necessary FBAR and FINCEN reports. Here are a couple of links you should read and make sure that you are in compliance with the reporting.