Under Part P of the Building Regulations, a new circuit, a consumer unit change and some work in bathrooms must be notified. An electrician in a competent person scheme can sign it off themselves; otherwise building control has to approve it. Small like-for-like repairs are usually not notifiable, and none of this is a job for DIY.
After the swap you get an Electrical Installation Certificate from the electrician, and their scheme sends a separate Building Regulations compliance certificate, which GOV.UK says arrives within 8 weeks. If building control approves the work instead, they issue a completion certificate. An EICR is a different document and does not cover new work.
Planning Portal electrical building regulations guidanceGovernment guidance on finding a registered electrician