I do not know another solution but tweaking around with two columns. Is there a reasons why you do not want to use two?
% arara: pdflatex
\documentclass[12pt]{report}
\usepackage{booktabs}
\usepackage{siunitx}
\usepackage{caption}
\begin{document}
\begin{table}
\centering
\caption{My caption}
\begin{tabular}{@{}S[table-format=4.0]@{\ }l@{}}
\toprule
\multicolumn{2}{c}{No} \\
\midrule
55 &(\SI{11}{\percent}) \\
47 &(\SI{9}{\percent}) \\
1 &(\SI{3}{\percent}) \\
1115 &(\SI{22}{\percent}) \\
36 &(\SI{7}{\percent}) \\
435 &(\SI{9}{\percent}) \\
123 &(\SI{25}{\percent}) \\
1438 &(\SI{29}{\percent}) \\
\bottomrule
\end{tabular}
\end{table}
\end{document}
This is quite easy, I think.
I have added the package caption
which gets the vertical spacing right. I would leave away the @{}
infront and after the columns, but this is a matter of taste.
The table still looks bad as the percentage numbers are not aligned correctly. I would prefer something of the following:
% arara: pdflatex
\documentclass[12pt]{report}
\usepackage{booktabs}
\usepackage{siunitx}
\usepackage{caption}
\begin{document}
\begin{table}
\centering
\caption{My caption}
\begin{tabular}{S[table-format=4.0]S[table-format=2.0]}
\toprule
\multicolumn{2}{c}{No} \\
\cmidrule(lr){1-2}
{Number of votes} & {Corr. in \si{\percent}} \\
\midrule
55 & 11 \\
47 & 9 \\
1 & 3 \\
1115 & 22 \\
36 & 7 \\
435 & 9 \\
123 & 25 \\
1438 & 29 \\
\bottomrule
\end{tabular}
\end{table}
\begin{table}
\centering
\caption{Summary of people saying "No"}
\begin{tabular}{S[table-format=4.0]S[table-format=2.0]}
\toprule
{Number of votes} & {Corr. in \si{\percent}} \\
\midrule
55 & 11 \\
47 & 9 \\
1 & 3 \\
1115 & 22 \\
36 & 7 \\
435 & 9 \\
123 & 25 \\
1438 & 29 \\
\bottomrule
\end{tabular}
\end{table}
\end{document}