Different types of plants need different fertiliser. For example, floating plants need liquid fertiliser while plants such as sword plants and cryptocorynes need tablet fertiliser inserted into the substrate.
As you are in the UK, the best fertilisers to look at are Seachem Flourish Comprehensive Supplement liquid* and Seachem Flourish root tabs; and TNC Lite liquid* and root tabs.
* Seachem make several products under the Flourish label, use only Flourish Comprehensive Supplement for the Planted Tank. TNC make two liquid ferts, look for Lite.