The British Library in London has declared the digital images in its Illuminated Manuscripts collectionto be public domain. 

The Catalogue of Illuminated Manuscripts content is now available for download and reuse. Although still technically in copyright in the UK (and a number of other common law territories) the images are being made available under a Public Domain Mark which indicates that there are no copyright restrictions on reproduction, adaptation, republication or sharing of the content available from the site. 

For full details, see the Guidance Notes at this link: http://www.bl.uk/catalogues/illuminatedmanuscripts/reuse.asp

Most of the illuminated manuscripts in the Harley and Sloane collections already have images uploaded into the digital collections. (The only major BL collection of illuminated MSS not yet digitized is the Cotton collection.) Note that the emphasis here is on the illuminations; most MSS have not been digitized in full, though you can check the separate collection, BL Digitised Manuscripts to see if they have what you’re looking for. (There is a good selection of Harley MSS in this latter collection.).