I don't necessarily find one easier to grow. Peppers are slower to germinate than tomatoes in general. Tomatoes usually come up in a few days to a week. Peppers are more like a week to three weeks.
We have somewhat different climates, so I don't know how much good my advice can be. My climate is more continental, where yours in Bristol is more oceanic and temperate. (In other words, I have a colder winter and a hotter summer.) My average frost date is also much later (pretty much now for my area, where yours would have been March). I start my seeds indoors, supposedly a 6-10 weeks before the last average frost (end of March, beginning of April), though I tend to be slow on this for (I was still sorting out what to grow and ordering more seeds the first week of April ...

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Anyways, I use a heat mat when my seeds are germinating. This helps keep the media warm and improves germinate rate and speed. Peppers like a germination temperature of around 70°F/21°C. (Tomatoes like a bit warmer but not too hot - you don't want to cook the roots.)
Cold can stunt tomatoes and pepper, so if it's been getting down below 50°F/10°C, then that would slow your plants growth and potentially inhibit fruit set. If it's staying about 40°F/5°C, it's usually not damaging to the plants, though, just a delay. The good news is that you don't need to worry much about temperatures getting too hot to set fruit, (Over 90°F/32°C can incite blossom drop.) and you have a pretty long growing season, so even if your plants are a bit delayed, you should have ample time to get plenty of peppers still.
We plant our tomatoes and pepper in containers for a variety of reasons (mostly critter related), so we frequently bring a number of our plants inside at the end of our growing season when it starts getting too cold and frosting. Fruit will ripen, albeit slowly, in cold but not set; frost will kill. (Our season is usually over by mid-October; you probably have a couple weeks past that before it's getting too cold to get much fruit and longer still until risk of frost.)
Basically, your pepper plants like similar conditions to your piggies, maybe a touch warmer, and sunnier. They also generally prefer rain water to tap water, which may be something they aren't getting depending on your greenhouse set up. You may need to adjust/add fertiliser depending on how the plants are being watered to assure they have the best nutrient ratios for healthy plants and increased fruit set. Some years are just not cooperative for growing though ... I know we've had years we've written off too hot, too cold, or just bad when trying to assess a new plant variety; the weather cannot be controlled after all.