At the moment only Canyon and Rose bikes have actually stopped selling to the UK. There might be other online retailers who have stopped delivering to Ulster specifically but that will be down to issues with the courier company that they are using. Eg i do know that DPD have cancelled all mainland pick ups that were destined for N. Ireland citing issues with new computer system designed to accommodate the new paperwork for a post-brexit n.ireland.
On the other hand Parcel Force have only cancelled deliveries to the Free State and not N. Ireland.
Give it a couple of weeks and new computer systems or change of courier company will probably mean that everything gets back to normal.