Oval at high altitude

I was in my garden enjoying the nice weather and looked at the sky as I laid back on my rug. I noticed an odd black oval shape in the sky travelling at speed; at a high altitude in a straight line. I watched it for approximately 1 minute until it went into clouds. Absolutely not an aeroplane as I am familiar with the shape of a plane, where I live we are on a flight path so I regularly see aeroplanes at high altitude so can differentiate between a plane and what I saw. I alerted my husband who was with me, at first my husband said is it a bird, we continued to watch but it’s behaviour was not consistent with a bird. It was travelling very fast, in a straight line and then disappeared into clouds. Too far away to get any detail other than an oval shape. No wings, no slip stream. Just very odd. I am not too sure of the exact time. I apologise that this report is so vague and a bit unremarkable.