Invoice class: how to change currency

The use of dollar signs is hard-coded in the template as \$. If you want a complete replacement of $ with £, you can add

\renewcommand{\$}{\pounds}

to your preamble.

\documentclass{invoice}

\def\tab{\hspace*{3ex}} 
\renewcommand{\$}{\pounds}% $ -> £
\begin{document}

\hfil{\Huge\bfseries Initech Inc.}\hfil 
\bigskip\break % Whitespace
\hrule % Horizontal line

123 Broadway \hfill (000) 111-1111 \\ 
City, State 12345 \hfill [email protected]
\\ \\
{\bfseries Invoice To:} \\
\tab James Smith \\ % Invoice recipient
\tab Generic Corporation \\ % Recipient's company

{\bfseries Date:} \\
\tab \today \\ 

\begin{invoiceTable}

\feetype{Consulting Services} 

\hourrow{October 3, 2012}{8}{12} 
\hourrow{October 4, 2012}{6.5}{12}
\hourrow{October 5, 2012}{5.25}{12}
\hourrow{October 10, 2012}{9.75}{20}
\hourrow{October 11, 2012}{5}{12.51}

\feetype{Accounting Services} % Fee category description

\hourrow{October 10, 2012}{2}{80}
\hourrow{October 11, 2012}{1}{80}

\feetype{Hosting Expenses} 

\feerow{Web Hosting: October, 2012}{60} 

\end{invoiceTable}
\end{document}

You need to edit invoice.cls and look for \$ and change it for whatever you want. Probably change to \pounds. Take a look at lines 62, 69, 74, 80 and 107.