In the UK, after completing a Compulsory Basic Training (CBT) course, you are granted a certificate that allows you to ride a motorcycle or scooter with a maximum engine capacity of 125cc and a power output of up to 11kW (14.6 bhp). The Aprilia RS 125 Replica GP is a 125cc motorcycle, so it falls within the permitted category for CBT holders.
For example, if you are under 19 years old, you may need to display L-plates on your motorcycle and cannot carry passengers until you obtain a full Category A1 license.
To ensure you have the most accurate and up-to-date information regarding licensing and legal requirements for riding the Aprilia RS 125 Replica GP after passing your CBT, I recommend contacting the Driver and Vehicle Licensing Agency (DVLA) or consulting an official government website, such as GOV.UK, which provides detailed information on motorcycle licensing regulations in the UK.