We have been informed that:

To support external candidates looking for a place to sit their exams, RHS Qualifications successfully trialled a host examination venue in London in 2017. This has proved increasingly popular, and we have therefore taken the decision to move from London to RHS Wisley Garden from February 2019, where we can accommodate a greater number of candidates.

If you want to find a centre please go to the RHS page which gives you full information on this.