I keep getting vaginal thrush prior each period since last 5--6 months and inflamation around the area,is it safe to use canenten pessary regularly?

Yes, probably safe: First, confirm the diagnosis with your doctor. If you indeed have recurrent vaginal yeast infection (thrush, candidiasis), the best preventive usually is fluconazole (Diflucan in the US, not sure about UK), a single pill by mouth once a week -- or in your case, maybe monthly. Canesten (clotrimazole) probably would work and safe as well. Discuss with your GP, or maybe your local NHS GUM clinic.