In Sweden similar trees are often sold as a potted plant for Christmas. Sometimes they use a silver paint on them and anecdotal (I was told this by a professional gardener over 20 years ago) that practice came from a grower that had gotten a disease that actually killed the plants. A little paint and he could sell the plants for a higher price instead of suffering a huge loss!
Yours however look somewhat paler than what is usually sold for Christmas as a green tree. This could be from growing conditions in the garden centre or perhaps there are newer varieties out that actually are house plants. However I wouldn't bet on it and my recommendation would be to bring your plants outside, at least for the summer. This way they would be a lot healthier and stronger if you decide to take them inside at a later time. Normally it's however recommended to grow these outside and that is what the grower does as well. In London you could probably be successful with having these in a pot outside during a normal winter.
Don't water on a schedule, the soil should never be allowed to dry out, but don't overdo it. Select pots with drainage holes if your current ones don't have any.
Indoors you will need to keep humidity up as they are sensitive to dry air and this will also cause them to go brown. You could try misting them. As far as I know they should really just be seen as temporary visitors on the windowsill.
