Aerospace is a big business for the UK. We have giants in our own right such as Rolls Royce (23,000 UK employees), BAe Systems, GKN, Martin Baker and dozens of other companies. In addition, there are numerous foreign companies who had manufacturing and R&D facilities in the UK.
In all the UK exports around £30 billion worth of Aerospace equipment and it supports nearly 1 million jobs. www.bbc.co.uk/news/business-44840954
Currently, all the parts made by UK based companies are certified by EASA (the European Aerospace Safety Agency). After Brexit EASA will no longer have authority in the UK and it will be up to the Civil Aviation Authority to handle all aspects of air safety including certifying all parts and equipment.
Now here comes the problem. The CAA does not have the staff, nor experience to do what the EASA currently do.
ADS, the trade association for UK aerospace industry, estimates it could take approximately 5-10 years for the CAA to rebuild its safety regulation capability to fill in those responsibilities which EASA currently holds.
Rolls Royce have recently chosen Germany over the UK to expand it's R&D facilities. It is now looking to shift for work and jobs to Germany so that it's engines can still be certified by EASA.
The Royal Aerological Society pointed out that
"Far from diminishing UK influence in global aviation,
the EASA regime has provided a conduit for UK
influence on aviation safety and security within Europe
and beyond on behalf of the UK passengers flying on
airlines around the world; indeed, the UK has been a
major driver of ever-closer alignment on regulatory matters across Europe"
If we do see hard borders and slow customs checks then this will be highly disruptive to the "just-In-Time" manufacturing methods used in modern industry, but the inability to have our national regulators able to certify products will be further damaging to an industry which has so many high skill jobs.
Airbus have already warned it may need to cut investment in the UK.
www.bloomberg.com/news/articles/2018-07-18/airbus-condemns-u-k-s-unraveling-brexit-plan-echoing-rolls