They will do the job as intended. And they don't look too bad either!. The trade off is worth it for most people. They are also cheaper to buy than their metal counterparts. You get more for your money. Some plastic examples, like the ones made by Jones and Son UK are very easy to snap down to the required size due to them having indents (breaking points) running along the base. See the video below for a quick example.
This makes fitting them suitable and adaptable for almost any space big or small. Plastic versions come in many lengths but the most common sizes are 3 Meter and 5 Meter. See the full range here. Bigger quantities are for sale, but the price does rise quite high from 10 Meters and above. Of course, if you have a lot of space that needs to be covered then you will need to shell out the extra cash for more meters. Twenty meters will cost you anywhere from £50 - £60.
I need some. Were can I buy these?
You can buy them online from outlets like Amazon and eBay, or you can go to any one of the well known hardware stores in the UK and easily purchase some. B&Q, Screwfix, Homebase, Lowes, you get the idea. They are widely available. Bird Spikes in general don't just do a great job of detering Pigeons, they also keep a lot of other birds away, of the larger variety, like Crows, Magpies and Seagulls. Anyone can fit spikes. It's a fool proof procedure. You can use a Silicon glue, screws, nails and cable ties to secure the spikes to the desired surface.
In the shops mentioned it's not guaranteed that you will see products made by Jones and Son being sold, so it would be a good idea for you to check out their products on Amazon as they come complete with Silicone glue included for fixing them to the location, a "How To" CD for anyone that needs a little guidance with setting them up, and also an impressive (Free of charge) 15 year warranty!