Hi,
Anything is 'realistic' if you have the funds
With listing buildings it'll be ongoing repair and maintenence that's likely to cost you significantly larger amounts of money compared to buying a 'normal' property. You'll be legally required to use approved contractors, materials and you need to ensure everything is done correctly, properly and with minimal changes etc. In short, you'll be preserving the building for it's listed status.
Here's a site which covers the above in more detail
http://www.bricksandbrass.co.uk/buying_period_house/buying_listed_building.phpI've never really looked up listed buildings or properties for sale, so I don't know if your budget is good enough to look for properties in the area you'll most likely settle in or move to.
Overall, it sounds as though you'll need deep pockets to maintain a listed property. If there's perhaps a flat within a listed property and the management company and your monthly fees cover the costs, then that would perhaps be a good compromise.
If you are looking for properties that have some land, then £300K probably won't get you much for the most part in London and the Herts/Beds area you mention. However, in other parts of the UK you may well find something with maybe 1/4 or a 1/2 acre perhaps 1 or 2 acres of land. If you can't find something like that, then perhaps a house with a garden will be 'enough' to perhaps do a single or double story extension on.
If you expand your search in the S Bed's area, you'll maximise the chances of finding something. It'd be less of a chance in Herts, and the least chance in North London.
Hope this helps.
Cheers, DtM! West London & Slough UK!