Mandevilla is tolerant of frosts down to -1°C, harder frost than that are likely to kill the plant if it is not given protection.
Flowers can be seen in the UK up to Christmas if the plant is given the correct conditions as an example please bring it in doors before first frosts and place in a well lit area free from frosts and an ideal temperature of min 5°C (42F), a heated greenhouse or conservatory is an ideal environment to ensure sure further flowering, large bright window, patio door ways and finally protected porch which must again have good light. Conservatories do not have to be heated, just ensure you keep the plant at the front of the room to ensure it gets as much light as possible, in our own conservatory in the winter 2010 without heat plants continue to survive even with outside temperatures going down to -15C in January, the temperature did go down to -4C inside but because no ice formed on the foliage and the critical point the plants were kept very dry . It is very rare if the plants will wilt.
The shoots will continue to grow through the winter months mainly due to the very low light levels and these can be pinched back if required, but discontinue this practice from late February as the new shoots will provide the new seasons flowers.
In the spring and early summer as the light and warmth improves increase the watering and the plant can be cut back if you wish to maintain a plant of similar size to the year before, remove any dead or decaying material and yellow leaves, the new growth will produce the new flower buds, so do not cut back to late in the spring.
In an unheated room/conservatory it is essential the plant is kept dry, allow it to dry out between waterings, wet feet it will not like during the dark and cold months, and we mean let it get really dry from late October onwards, just give one cupful of water per month, and none if the compost is still moist/wet.
Further information is available on our Mandevilla FAQ page.