I love the UK so to me there would be few downsides about that duration.
If faced with the issue, I would be considering where and when -where as in where to use as a base or bases - part(s) of the country, city, village, 1 month in 3 places etc. When would be the time of the year - the weather is less than great a lot of the time.
I would definately go in 'summer time' when the days are longer - but avoid some of the peak summer months perhaps. I would choose April, May and June if it were me.
I think you would be in a good bargaining position with long term rentals.
Do you plan to have a car for all, some or none of the time - as this would be a factor in where to stay. The UK is expensive, so you may need to budget carefully.
The British are by nature quite reserved - so if you are going alone, bear this in mind. To me, that could possibly be the only downside. On the other hand of course, they speak English, which makes communication easier.
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