As a consumer in an EU country, you have various options for complaining outside of the court system in cross-border cases. It is still not clear if you will have these options in the no-deal scenario.
In that case, the opportunity to complain through the ODR platform will end after March 29, 2019.
The UK European Consumer Center
The UK ECC is currently expected to continue until March 2020. However, there is no certainty that they will continue afterwards. You can read more on their website.
The approved, British ADR’s (Alternative Dispute Resolution) are likely to continue after Brexit. It is, however, still not certain that consumers in other EU-countries will be able to submit complaints against British traders using these agencies. The ADR’s are based on EU law.
In the longer perspective, the British government will decide whether this option will still exist.