If your cylinder has a red jacket, then you will be wise to replace the cylinder as current cylinders have better insulation and bigger coil to heat the water quicker. But if you want a dry loft, you may want to look at unvented cylinder.Red jacket cylinder will lead to erratic operation if coupled to new boiler. Also, heat loss from cylinder, while OK for drying washing, is unacceptable.
Rule of thumb.
Red jacket cylinders needed 3kw allowance. If radiators totalled to 12kw, your boiler would have been 15kw minimum. In most cases, boiler would have been slightly bigger then adjusted down to 15kw.
Modern cylinder coils are bigger (already said), with unvented cylinder coil being massive (rated around 24kw). 15kw boiler will be working full time to heat the cylinder. Once the cylinder is hot, then boiler can be used to heat the rads. Cylinder heat up time is extended as it would not be getting the heat it can handle
Modern boilers are like cars with cruise control. When you resume cruise control, car picks up speed to reach the set speed and then cruises at that rate. Similarly, boiler will run at max to reach the control set temperature and then (in most cases) go to lower burn rate to match the heat output to satisfy the load (radiators or cylinder)
Answer simplified for sake of explanation