Frequent service dropouts

I've signed up with Smarty and pretty appalled at the quality of coverage. 

All throughout London my signal constantly drops out completed, showing zero connection. This is inside and outside, in congested and non-congested areas. And often if I quickly put my phone onto airplane mode and then back on again, it finds a signal. 

I was recently overseas and the connection was strong and constant so I know its not my phone or the sim. 

Why is the connection so terrible and is there anything I can do to fix this?