I'd keep kitty indoors and permanently close the cat flaps.
However, did your vet discuss with you the administration of any meds? I find it strange, because Struvite crystals can be dissolved with an acidifier - our vet always prescribes Albeta. Of course, this doesn't help with a blockage - kitty under these circumstances does have to be catheterized.
But the diet is VERY important for keeping the crystals gone. Our kitties were put on a Science Diet C/D diet, which is working for them. However, a diet that is low on grains should work too - and adding a cranberry supplement will also help.
To help relieve stress, you can try purchasing Feliway and Calm & Serene (or Rescue Remedy) Flower Essences. These help some kitties - some kitties they don't. But Feliway is a synthetic hormone that mimics the "friendly" markers in cats' cheeks, and usually helps. I don't know where to obtain those in England. Here our vet sells the Feliway, though it's available in most pet stores now.
I don't know if you can order from this site, but this is where we get them: http://www.catfaeries.com
Also, just extra attention and play time can also help diffuse stress.
Helping him drink extra liquids is also good - keeping the bladder flushed. Put out more bowls of water. Strangely, we found that our kitties are drinking more out of black water dishes than white or metal ones. ?????????? Don't know why, but there it is. We also have a drinking fountain for them, one on each floor of the house. We bought one that pours out a spout rather than running down a ramp. They LOVE it. Here the brand we bought is Drinkwell, though I don't know what's available to you there.
And while this is controversial, we only give our kitties distilled water to drink. Maybe a filter attached to the sink would work - or look for a spring water that is low in magnesium and calcium. Distilled water supposedly leaches minerals out of your bones - but our vet says that this would be offset by the nutrition from the food, and because we've had three male cats catheterized, two of them twice, we're not taking any chances. So we give them distilled water, because we know we have very hard water coming through the tap.