Simple as it looks. 1-hour chart on GBP/USD showing RSI at the 70 level level, suggesting it is now overbought for the current time period. This is especially true as the pair retests the 1.265 - 1.266 resistance level, hence making it harder for there to be a strong breach above the 1.266 level. Thus there being a likelihood of the pair heading back down to 1.255. However, I do not think it will easily reach 1.25000 or even anywhere below that based on the most recent developments - the Oxford vaccine is big news, and with the macroenvironment really improving over the Covid pandemic, safe-haven demand for assets like the USD will drop. So, PAY ATTENTION to USD weakness/strength as that will dictate the next possible moves.
Note: This is not professional advice. Please trade at your own responsbility - I will not be held accountable for any losses.