I am looking for monthly composite NDVI data for the UK between 2000 and 2013. The resolution is not so much of an issue. I am struggling to find relevant data and images for further processing in imageJ / ENVI.
You have MODIS NDVI composites for 16-days and 1 month period. Is the most frequent NDVI data, but the start date is from February 2000, so you can expect to have all data from the desired period.
You can access to data from Data Pool or Earthdata, in the last one you can preprocess scenes to reproject, mosaic, crop scene, select bands and change image format prior your needs for some products (Customize Product tab).
Also, you have different NDVI 8 and 32-days composites from Landsat Data and processed by Google. You can use Google Earth Engine to create time-series over UK and download them to work in ENVI or imageJ