I'd look into getting a very cheap cloche, coldframe or pvc greenhouse if you possibly can. Last year (about this time of year) I bought a 3 tier pvc greehouse and a very, very cheap coldframe (the frame was about £30, the greenhouse about £16). To say how cheap they were, I am really pleased with the coldframe, and think it will last a good few years. I used them in spring to protect some plug plants, and all survived the late frosts we had up here in Yorkshire.
In very cold weather, I put some polystyrene sheets under the trays with the pots in, and put the whole lot in a sheltered corner of the patio. This had the added bonus that on milder days I could open the top and let some natural rainwater in, and didn't have to harden them off as I would have had to do if they'd been in a heated greenhouse.
Coldframe came from Amazon, PVC greenhouse from ebay, and another 2 places to look for inexpensive garden stuff is Poundstretcher and Wilkinsons, I'm sure you'll have at least one of these near you. Sometimes Aldi or Lidl have walk-in greenhouses (£30 last year), but it's the wrong time of year to look. Can you tell I'm used to gardening on a very tight budget??