Two things must happen. First, existing wildflower habitats need nurturing to ensure they are maintained. Second, we can start to put back wildflower-rich habitats.
It is vital that the remaining wild meadows are maintained by managing them appropriately, whether they are in towns or on farms in the countryside. Much of my work involves providing advice on cutting and grazing, which are vital to maintain wildflower grassland. Without management, the grassland wild flowers disappear to be replaced by tussocky grasses, sedges, scrub and trees. Too many grazing animals can also ruin wild meadows by damaging the soil and encouraging thistles and other weeds.
It is also possible to create new wildflower grassland, although this is never as rich and diverse as an ancient downland or hay meadow. I also offer advice on how to do this - but every site and circumstance is different. One size does not fit all (i.e. location - soil, slope, management possibilities - will affect the plant species to sow that will survive). I can give advice on methods of establishment and provide specifications for cost-effective seed mixes that can be used. Creating new wildflower grassland does not need to be expensive, but care needs to be taken in selecting the correct species. Be warned that many seed merchants will suggest sowing at an un-necessarily high rate and may recommend seed mixes containing unsuitable species or agricultural varieties (rather than British native-origin stock). A sowing rate of no more than 15kg/ha is nearly always sufficient.
If you want to do your own research, in my role as Technical Adviser to Flora locale (the wildplant restoration charity), I have written and co-written lots of information which is available via the Restoration Library at www.floralocale.org.