Permitted Development Rights changed in 2008. (and several times since)
You can now erect detached outbuildings (ancillary to the domestic enjoyment of the dwelling) to up to 50% of the area of your garden, so long as nothing is built forward of a principal elevation of the house which fronts a highway and the maximum eaves height is no more than 2.5 m above adjacent ground level. Maximum height must be no greater than 3.0 metres with a flat roof (10º or less) or 4.0 metres with a pitched roof (>10º), again both measured from adjacent ground level. If however any part of the outbuilding is within 2.0 metres of a boundary, the maximum height reduces to 2.5 metres.
The above is on the basis of the house not being Listed or within a Conservation Area or area covered with an Article 4 direction, where in each instance a planning application would be required.
If your budget stretches to it, go masonry. Besides, it will support a nice lifting beam across the middle
You need to check the minimum pitch requirements for the tiles you have; if they are small format clay, it will be 35º, which will impose geometrical limitations on the size of the garage whilst still following the PD rules above. Concrete interlocking go down to 17.5º
The weight of the tiles is easily overcome by appropriate design.