If you are simply having a like-for-like swap in the same location then it is likely that you will already have a power supply in place for your new boiler.
However it is often beneficial to relocate the boiler during the upgrade works in order to give more room within the property. This can often be into a cupboard or even the loft. These locations are out of the way, however generally do not have a readily available power supply for your new boiler.
This is where using an electrician is essential to complete your boiler electrcial installation.
Power can be sourced either from a locally available socket via a fused spur, or more ideally on it's own dedicated supply circuit for the boiler.
Having a dedicated supply circuit ensures that your boiler (and thus your heating & hot water) is not affected by issues with any other circuit. For instance, if your socket circuit was to trip or turn off, your boiler would still have a working power supply.
A dedicated supply circuit for your boiler should only be installed by a qualified and experienced electrician. In England, a new circuit is 'notifiable work' under the Part P of the building regulations. Hence this work should really only be completed by a registered electrician who can self-notify their own work.