I wouldn't fit a boiler in a loft I know it's perfectly acceptable but think about it its freeing cold up there in the winter, boiling hot in the summer boiler components hate temperature extremes, the regs state a permanent walk way and hand rail ,fixed ladder, wired in light, isolation of the water and gas out of the loft , and a means of filling and monitoring the system pressure again out of the loft , no way of adjusting the boiler controls without getting in the loft , freezing condense pipework is a risk, any leakage will damage the fabric of the home and possibly flood it ,should the boiler fail and need a manufacturers repair many will refuse if it's installed in a loft so many negatives ? there's always a alternative.
Worcester Combi boilers are no better or worse than any of the other top makes in my opinion. Cheers Kop