Unfortunately the probability of a no-deal Brexit is more than high. And this would have direct impacts on British citizens living and working in Germany and their employers and vice versa.
A no-deal Brexit will legally transform the United Kingdom into a third-country. The EU freedom of residence and employment will end on March 29th, 2019. Therefore British citizens will require a residence and work permit in order to live and work in the EU (including Germany) and EU citizens require a work and stay permit for the UK.
Good new is that the German Government is planning a three months transition period after March 29th for this no-deal scenario. During this period it will be still legal for British citizens to live and work in Germany. They need to request a stay and work permit and have their residence registered in order to remain in Germany after the end of the transition period. Some (but not all) local foreigner authorities in Germany are already offering registration options.
EU Employers of British citizens need to consider this change and assure that their employees have valid stay and work permits as otherwise they must not be employed in EU (including Germany) or UK.
Basically the same applies for EU citizens (including Germans) which are already living and working in the United Kingdom. They may now request an unlimited stay permit according to the EU Settlement Scheme. All EU citizens planning to move to the UK after a no-deal Brexit require a visa and a work permit.
In case of an orderly Brexit the EU freedom of movement will stay effective for a transition period until December 31st, 2020. The draft of the Exit Agreement stipulates an extensive preservation of the freedom right for British citizens and their families which are already living in EU/Germany. Nevertheless the further continuance in Germany after December 31st, 2020 also requires approval by the German foreigners authority.