Tides can be shown either in GMT or BST, though, as the rules for setting BST can change at three yearly intervals, you'll need to be cautious with this option. It's almost impossible to check the accuracy of tidal predictions. Even if you turn up at the port on the right day with a tide gauge, the height may have been affected by weather or atmospheric pressure. Together with the UK Hydrographic Office, Proudman is amongst the most respected producers of tidal information. They maintain the national tide gauge network and provide information to the Environment Agency - so a high standard of accuracy is expected.
Any differences are unlikely to be significant for practical boating purposes.
Sunrise and sunset predictions cover a whole year and are in GMT only. In essence the data is no more than you can get from a nautical almanac except that you are spared the drudgery of calculating tide times and heights at secondary ports. If your sailing is exclusively round the coast of the British Isles, the program is worth considering but the lack of any information about continental ports restricts its value for the more adventurous.